Classic Spike Pit: A trap door opens up below the character. Roll 1d6 for the number of spikes landed on, and then 1d4 damage for each. Optional: 50% chance of poison for each spike, with character losing 1 HP per minute until cured or dead.
Classic Snake Pit: A trap door opens up below the character. Roll 1d4 for number of snake attacks. Roll d100 again for each snake: 01-75% snake is poisonous, 76-00% snake is constricting (consult favorite monster guide).