20 Pieces of (Hopefully) Strange Dungeon Decor

The goal of this list is to be weird, sometimes ominous, but not immediately hazardous.

I'd use this to provide tangible hints of a room's history (or perhaps at what the whole dungeon was originally built for) and overlay a "what the current faction uses this room for " table.

1. Ancient scorch marks and glassed rock surfaces. Whatever took place here weakened the barrier between worlds. During an eclipse, the barrier weakens further, and the various dungeon factions will temporarily truce and set up market here to trade with extraplanar visitors.
   
2. A copse of transparent-needled pine trees, growing in an unseeable light. Spending significant time here (e.g. camping overnight) runs the risk of radiation poisoning.
3. Scrawled-on parchment scraps joined by red twine are affixed to every square foot of wall. A player can pose a question about the dungeon and have their character spend 2 turns (20 minutes) sorting through these notes for a 1-in-6 chance of a correct answer, and a 1-in-6 chance of an erroneous answer.
   The author's bones, picked clean by flesh-eating beetles, lie in a crevasse elsewhere. If his journal is recovered the odds of a correct answer increase to 4-in-6. The walls hold enough information to answer six questions, however long that takes.
4. A ghostly ecosystem drifts through the room. It contains 1 metre wide, 10 metre long jellyfish, small fish, miniscule zooplankton, and bacteria visible only as a shimmering ghost-sediment. At present, they are intangible and harmless.
5. A clockwork well constantly generating a lot of noise. If cranked, it will dispense precisely 1 litre of water. If oiled, will generate much less noise for about a week.
6. Metal filings cover the floor. They stand on end, and are repelled by iron or steel.
7. A tiled pool filled with tepid, shin-high water that is potable and magically self-cleaning. Dungeon monsters bring their toddlers here for swimming lessons.
8. Detailed, life-size origami effigies of the player characters stand in the centre of the room. Pulling a lever on the wall will cause the effigies to refold into whatever creatures will next walk into the room. Creatures who are aware of this mechanism's existence become immune to its powers.
9. Yellowed, curling WW2 propaganda-style posters warning denizens to be ever-vigilant of water weirds (or some other "trap" monster).
10. Twenty meathooks holding clay sculptures of animal carcasses. Some are realistic, some abstract, some naïve.
11. A stream of ants marching between two crevices, carrying thin scraps of gold leaf (1,000 scraps = 1 gp) from some unseen treasure vault to another unseen room where the ants farm their gold-metabolising fungus.
12. An automatic tapestry loom dominates the room. It slowly, in real time, documents the history of the local surface-world lords above. Go on, cut a thread in active use, just try it.
13. Crystal egrets fish in a lake of unknown clear liquid (harmlessly passes unabsorbed through the system of any human who drinks it). If exposed to sunlight, the egrets will grow sickly and crumble to dust in the course of a few days.
14. This room has no flat surfaces, and is really a void in an irregular packing of chrome spheres of various sizes. Each sphere is worth its weight in silver, but may cause a cave-in if shifted.
15. A low ceiling covered in cloth sharply transitions some 40 ft into the room to a much higher ceiling out of view. The low-ceiling area is littered with human-size mousie toys.
16. Prominent stone wall, carved with a phylogenetic tree of all demihuman and humanoid species known to the characters, plus several unknown ones. This might be multiple trees if the setting history doesn't have descent from a common ancestor. One entry is chiseled out, but the stone is extremely hard and resists common hand-tools.
17. A nanosun about 1 metre in diameter hovers in the centre of this roughly spherical room. It provides bright reddish light, local antigravity (one can walk anywhere on the sphere's interior), and the occasional lashing streamer of solar wind.
18. Huge excavated tree roots with taps installed -- the sap is delicious and nourishing, but pleasantly dulls perception and is partly antimagic. Drinkers are resistant to spells (including beneficial ones) and have difficulty casting spells, these effect scales with dosage.
19. Immobile electrical apparatus fills most of the room, with a prominent terminal featuring numbered dials and a gramophone horn. Racks of large carved stone ears line the walls, each ear is 2 metres long and carved with a serial number. Dialing a number into the apparatus lets you listen through the corresponding ear over any distance.
    A random number has a 1 in 6 chance of connecting to one of the ears in the room, a 1 in 6 chance of connecting to an ear in some random nearby location, and a 4-in-6 chance of being an unassigned number, connecting to a buried ear, or being otherwise useless.
20. This whole room is a time-capsule buried by child godlings, It features 20 ft tall trading cards, an enormous packet of expired gum that now only grants *delusions* of godhood if eaten, amber marbles with humans imprisoned inside, and a really interesting stick. Disembodied choral harmonies are faintly audible at all times. Reading the cards' divine language aloud risks wild magic effects.
    The really interesting stick is quarterstaff-sized, and can be brandished as an action to transfix all creatures within 30 ft of you. A Paralyse/Petrify or Wisdom save negates the effect, which otherwise functions as Hypnotic Pattern (see the Illusionist spell in AD&D 1e or OSRIC, or the D&D 5e spell). The stick isn't magical. It's just really interesting.