1. Spikes, because an extra 1d8 damage is always a good standby.
2. Pick your favorite slime.
3. Oil, coupled with those ignition sources carried by the characters....
4. Rats. Lots of rats.
5. A pair of hungry rust monsters, ready to add insult to injury.
6. The remains of the last party that fell. And a carrion crawler.
7. As 6, but other random scavenger(s).
8. Frictionless floor. For the pratfalls.
9. Area floods with water after 1 round.
11. Ball pit!
12. Reanimated dead of the last fallen party.
13. Injured survivor(s) of the last fallen party.
14. Amorous Ogre.
15. Sarlacc pit. Because slow digestion.
16. Pick your favorite fungus/mold.
17. Asshole fairies who laugh at you and steal your stuff.
18. Big opportunistic spider and web.19. Spinning blades, because life should be more like a Bond film.
20. Pool of piranhas (because 19.)
21. Jello (or gelatinous cube, your call)
22. Engulfing sand.
23. Purple Pile of Putrescence
24. Snakes. Why'd it have to be snakes?
25. Pillows and kittens.