1) A chest in the centre of a room is bolted to the floor, with a clearly steel cable running from its lid into the ceiling. Opening the lid of the chest or cutting the cable causes the floor to drop and steel shutters to drop over the entrances- only a few feet down and with the doors only partially shut if the lid is merely opened, but far enough to take falling damage and completely seal the entrances if the cable is severed. Either way, the dropping floor should reveal vents by which some noxious or dangerous substance enters - poison gas, acid, yellow mold, green slime, or even merely large amounts of water.
PCs could know there will be trouble if they strummed or tapped the cable, which would cause the metal parts to rattle up above, if they checked the door arches after opening them to spot the shutters, or closely inspected the floor for seams, which would tell them parts of the floor were not connected to others. The trap could be reset by two strong people closing the lid's chest if the cable wasn't cut, or by stringing a new cable through the gearing up above and reattaching it to the lid below (a few hours work for an engineer if the materials are on hand, plus the same two strong people).
This trap should be fairly obvious, and is meant to signal an area as dangerous. It could be reset simply by refilling the reservoir of oil near the top. I think it works best in rooms with multiple entrances leading off into looping corridors, to increase the risk of a wandering monster chasing the PCs back through it. You could make this more difficult by adding a second trap (blowpipes in the walls firing knock-out darts or something) but that makes it even more likely to be a TPK.
3) Slats in the floor are actually triangular wedges that rotate freely but only in one direction. Stepping carelessly traps the foot and deals mild damage. It's possible to free someone without dealing more damage to their leg, but this is a noisy and slow affair, taking 1d4 turns of dismantling the slats, easing the foot out, etc. Each turn requires a wandering monster check. Wrenching the leg out will cause significantly more damage, possibly even crippling it until healing magic or surgery can restore it.
This trap can be noticed if the PCs are checking the floors, since the long thin slats made of metal should be distinct from other types of floors. The trap can be reset simply by rotating the triangular wedges back to being flat once the obstructions are removed. To make the trap nastier, you might want the space below the slats to be filled with serpents or green slime or another threat. I've found that acid and lava tend to make the trap too boring, since they either kill the PC right away from damage, or burn off the leg quickly enough to make the entrapment irrelevant.
The orange pillar shows a man bleeding from numerous spear wounds. The green shows a man smiling as waves of energy radiate from him. The purple shows a man vomiting. The brown pillar shows a man holding his arms up to heaven, smiling as he flies upwards.
Mixing the correct colour combinations of the primary-coloured goop produces potions. Only colours matter, not proportions (brown is produced either by mixing all three primaries, or by mixing any two differently-coloured secondary-colour potions). Orange produces potions of Cure Light Wounds, green produces hallucinatory potions of light (you glow, attracting more wandering monsters and being unable to sneak, while also tripping balls), purple produces emetic potions of Neutralise Poison. Brown produces instant death unless a save is made.
I recommend limiting the number of doses of each gunk to prevent the PCs from making off with hundreds of potions of Cure Light Wounds once they figure it out. I haven't experimented with this variation yet, but I think it might be fun to make the baseline potions weak (i.e. Slow Poison instead of Neutralise Poison for purple) and then require multiple doses of the same potion to be crafted and combined to get the stronger effects (so two doses of orange becomes a Cure Moderate Wounds potion, etc.). I also find the more happy you describe the green and brown pillars, and the less happy you describe the purple and orange ones, the more likely the PCs are to poison themselves. If you have cautious PCs, it might help to make the goop into incense, and the basin a brazier that fills the room with smoke that affects all of them at once.