Game-enhancing powers, game-ruining powers, and yet more magic items

I've been running OSR D&D more-or-less weekly for over five years, now, with a heavy focus on exploration, problem-solving, and diplomacy. Combat happens, but I learned early on that the kind of combat power-ups that most later D&D editions obsess over were almost irrelevant: when most fights are either one-sided ambushes or desperate fighting retreats, the shift from 1d8 damage to 1d8+2 damage is really not that big a deal. My players regularly forget which magical weapons their PCs are carrying. What they never, ever forget is exactly who is wearing the Ring of Invisibility to Undead. 

FORGOT WHO WAS WEARING THE RING.

Whether in the form of magic items or spells, I've tended to give my PCs access to lots of utility magic, in order to enable the kind of improvisational problem-solving on which such games thrive. But there's utility and then there's utility, and a power with too few limitations can quickly break a game. Previous games, for example, taught me the hard way that giving PCs high-speed at-will flight is a genie that's really hard to get back in its bottle once released, trivialising everything from difficult terrain ('I fly over it') to tripwires and pressure plates ('I fly over it') to melee-only enemies ('I fly over it and kite it to death'). And the Team Tsathogga game taught me that even the humble Charm Person could become hard to manage once the PCs had enough spell slots. ('We hide in the bushes and cast Charm Person on him twelve times! One of them's bound to work!')

Of course, these problems are context-dependent. A more superheroic game could probably have taken both flight and mind control in its stride. (Although maybe not: every Exalted game I ever played in ended up crashing and burning due to the difficulty of challenging PCs with near-unlimited powers of mobility, persuasion, information-gathering, and stealth.) But assuming you're after something resembling traditional fantasy, in which 'a castle' or 'a troll' is a dangerous but solveable problem to a clever and well-equipped party of adventurers, then here are my notes on the kinds of powers that make for good, enjoyable problem-solving tools, as well as the kind that can easily spoil things for everyone unless counterbalanced by significant downsides or limitations, and a handy list of magic items to go along with both... 

The Good Stuff - abilities that facilitate creative and intelligent play

• Levitation: Slow, vertical-only flight. Allows for all kinds of ingenious problem-solving but requires careful set-up, not particularly useful in combat, and generates hilarious mental images, especially if you allow levitating characters to be moved horizontally by party members pulling them along on ropes from below!
• Invisibility: This might seem very powerful, but if it genuinely is only invisibility - meaning that you still make sounds, leave scent trails, make footprints, etc - then there are so many potential failure points and counter-measures that it's more likely to be used as an ingenious component of a cunning plan rather than as a slam-dunk victory condition. Partial invisibility (e.g. you still cast a shadow) is even better. 
• Illusions: The power to create intangible visual and/or audible illusions enable more demented ingenuity than just about any other ability. Endlessly flexible, and the lengths that PCs will go to in order to make sure no-one reveals the trick by just touching their illusionary treasure / monster / whatever are the stuff that truly insane PC plans are made of.
• Tunnelling: Magical dig-through-stone-walls tunnelling can often short-circuit scenarios, especially in dungeon environments, but having a character with the ability to burrow through sand and loose dirt at semi-realistic speeds opens up all kinds of unorthodox approaches to break-ins, getaways, etc. 
• Light and darkness: These are good, flexible powers with a wide variety of uses relating to stealth and detection, most of which require a bit of set-up if PCs are going to get the most out of them. (If you're going to use darkness to cover your escape, how will you see while inside it?)
• Disguises: Powers that let people disguise their appearance to match someone else's are a particularly interesting form of illusion, because the potential pay-off is so large but the dangers of detection are so high. I feel that these are best when they perfectly mimic appearance but only appearance: when it comes to mimicking voice, gait, mannerisms, knowledge, etc, the PCs are on their own!
• Movement effects: Things like Jump, Spider Climb, etc - powers that let you get into places you normally couldn't reach. Allow problems to be approached from unusual angles, often quite literally. 
• Environmental manipulation: Things like creating heat, cold, fog, rain, etc - not inherently beneficial in and of itself, but capable of creating a new situation which cunning PCs may be able to turn to their advantage!
• Construction abilities: Essentially a subtype of environmental manipulation - the ability to e.g. rapidly reshape earth, construct barricades, etc. Anything that lets PCs reshape an area on the fly and thus shift the spatial dynamics of an encounter. 
• Water breathing: Like tunnelling, water breathing abilities open up new opportunities for getting into and out of places, and allow locations to be connected together in new ways via underground rivers, water pipes, etc. 
• Enhanced senses: Things like enhanced hearing, the ability to see perfectly in poor light (not total darkness), the ability to follow scents like a bloodhound, etc, opening up new and unexpected avenues for acquiring information. 
• Communication abilities: The more things your PCs can interact with, the better. Let them talk to monsters. To animals. To rocks, plants, corpses, water, air... As long as whatever they're talking to is not guaranteed to be useful or co-operative, giving them more opportunities for communication can only enhance their options for creative problem-solving. ('OK, how do we bribe the trees into helping us?')
• Temporary intangibility: The trick is to not pair this with invisibility, by either making it ghost-style phantom projection, or literal gaseous form. It then creates an interestingly asymmetric situation - you can see and be seen, but can't affect or be affected by anything you're seeing. Can be used either for scouting or for one-way infiltration - you can walk through the walls to get in, but then how are you going to get out?
• Very specific immunities: Complete immunity to fire, for example, or to falling damage. Abilities like these may be intermittently useful in combat, but they also allow situations to be approached in completely different ways. ('So I'll fly over the castle in a hot air balloon, jump out, drop a thousand feet down into the courtyard, and then open the gates from the inside...')
• Limited telepathy: Things like the ability to sense someone's emotions, or read their surface thoughts - just enough to give the PCs an exploitable edge in social situations without having to worry about every mystery collapsing on contact.
• Emotion control: Genuine mind control can easily become an 'I WIN' button, but the ability to scale up a specific emotion requires much more care to use effectively. 'I can make people feel really angry' is not, in itself, likely to solve many problems, but can easily be a component in such solutions...

The Bad Stuff - abilities that short-circuit play without significant limitations

• Unlimited flight: Trivialises too many kinds of obstacles and opponents, especially if it comes with perfect manoeuvrability as well. If you want to give your PCs access to flight, try to build in some serious limitations. 
• Unlimited intangibility: Genuine walk-through-walks-style at-will intangibility tends to trivialise information gathering, infiltration, escape, theft, etc - everything except combat, essentially.
• Mind control: This includes 'super charisma' powers of the 'I'm just that persuasive!' variety. Anything that means PCs can simply steamroller interactions with NPCs rather than having to actually work out how to befriend or manipulate them is likely to lead to much less interesting play. 
• Mind reading and lie detection: Short-circuits any kind of investigation or mystery and makes diplomacy much less interesting. 
• 'Sniper' attacks: Very powerful, very accurate, very long-range attacks can make for very one-sided, non-interactive combat scenes, and thus for very boring gameplay. This can easily become an 'everything looks like a nail' situation. 
• Unlimited information effects: The ability to commune with near-omniscient beings, for example, or powerful divination effects that always return accurate answers. It's usually OK if PCs are allowed just one question, but if this is a power they have repeat access to then it becomes very hard to maintain any kind of mystery, or even ambiguity. 
• Speed/mobility effects: When confronted with any kind of enemy, one question the PCs are always going to ask is 'can we just kite it to death?' This is a fair question, but if the answer is always 'yes' then the temptation to resolve every possible encounter in exactly the same way becomes very strong. I'd thus advise caution in granting mobility effects that allow PCs to engage opponents without ever allowing their opponents to engage with them: speed effects, for example, that let them move at full speed while still attacking. It's fun the first time, but it can become very boring very fast.

1d20 magic items for ingenious problem-solvers

1. Mat of Levitation. Anyone sitting cross-legged on this threadbare prayer mat can levitate at will by concentrating. If they stop concentrating for any reason then they fall. Only vertical movement straight up or down is possible. 
2. Ring of Near Invisibility. Anyone wearing this ring becomes invisible, but still casts a shadow. The ring itself does not become invisible, and may be spotted floating around by alert observers.  The ring does nothing to mask the wearer's scent, sound, or footprints.
3. Amulet of the Mole. This amulet permits its wearer to dig tunnels through sand or dirt (not stone) like a mole, at a rate of 5' per minute. Tunnels will only be wide enough for the person who dug them to crawl through on hands and knees. It does not grant any ability to see in the dark. 
4. Hatpin of disguise. Prick someone with this pin hard enough to draw blood, and the next time you place it in your own hat or hair you will take on their physical appearance until it is removed. (This is a visual illusion only, so if e.g. they have a beard and you don't then anyone touching your chin will realise something is amiss.) Your clothes and voice remain unchanged.
5. Armbands of the ape. Anyone wearing these chunky brass bangles gains the ability to climb and brachiate like an ape or monkey, rapidly scampering up trees or walls and swinging easily from branches, ropes, chains, etc. Wear them as anklets and you can swing by your feet, instead. 
6. A random weather bag.
7. A random emotion bag.
8. Tree Phrasebook. This enchanted book allows you to talk to trees... sort of. Reading from it gives you enthusiastic-tourist levels of ability to understand and be understood by trees, mostly by making alarming groaning noises with your throat. It does not make trees inherently well-disposed towards you, though they can be bribed with fertiliser or threatened with fire. Similar books may exist for communicating with rocks, rivers, etc. 
9. Thought interceptor earring. If anyone within your line of sight thinks a really big thought (e.g. 'OH MY GOD I LOVE HIM SO MUCH' or 'I WILL FUCKING KILL HIM') while you are wearing this earring, then you will 'hear' it as though it had just been shouted into the ear to which the earring is attached. Such thoughts are 'spoken' in the wearer's own voice and it will not always be apparent whom they are coming from, though context will often make this obvious. If you're in a whole crowd of people all thinking really big thoughts (e.g. a panicking mob trying to escape a fire) then the effect is simply deafening. 
10. Torc of water breathing. This golden torc is decorated with engraved gills. Wearing it allows the wearer to breathe underwater. It does not provide any swimming abilities, or the ability to see in the dark. 
11. Cloak of limited flying. Once per day, this bright red cloak permits its wearer to fly rapidly for 1d10 rounds - this ability is triggered by simply leaping into the air. The user is unaware of how long the duration is, and will only know the magic has stopped when they start dropping out of the sky. 
12. Ghost juice. Drinking this potion causes you to temporarily die and become a ghost for 1d6 hours. During this time you are intangible, allowing you to float around and walk through walls, but you are not invisible, and are clearly recognisable as a ghostly, ghastly version of yourself. You can talk during this time, though your voice is thin and tends towards wailing. You are immune to non-magical damage for the duration, and cannot cause physical harm to anyone else. At the conclusion of the effect you are sucked back into your body and return to life. 
13. A double-edged potion.
14. Divine Pass Note. Write a single factual yes-no question on one side of this enchanted papyrus, wait five minutes, and turn it over. God will have written YES or NO on the other side. Shortly afterwards, the note will spontaneously combust. Anyone trying to use this item to learn things that Man Was Not Meant To Know will find that they spontaneously combust, instead. 
15. Acoustic Amulet. Wearing this amulet enhances your hearing tenfold, allowing you to listen in on conversations thousands of yards away. Any kind of loud noise that occurs nearby while you are wearing them will be painful if not deafening. 
16. Hammer of the survivor. This worn construction hammer is stained with zombie blood. As long as suitable construction materials are available, it allows stockades and barricades to be constructed at twenty times their normal speed. 
17. Magic Feather. As long as this feather is gripped tightly, no fall from any height will cause any damage to the holder. 
18. FX Box. Looks like a complicated wind-up music box with a directional speaker. A wheel on the side can be set to any one of dozens of options (e.g. 'thunder', 'sounds of battle', 'screaming', 'birdsong', 'suspicious conversations', etc). When the crank is turned, the selected sound will be heard emanating from the place that the speaker is pointing at for as long as the user carries on turning the handle.
19. Black Breath Choker. Wearing this onyx choker allows the wearer to exhale huge clouds of blinding darkness, which completely block all light and dissipate like smoke (meaning that they'll vanish much more quickly in a high wind or similar). Each exhalation exhausts its power for 1d6 minutes. The choker does not grant the ability to see through darkness.
20. A bag containing 2d4 items from this list.