An apparently classic snake pit. You know the drill: something triggers a trap door, or the floor was illusory all along -- whatever it takes to get a player character down a pit filled with spikes and snakes.
Roll 1d6 for the number of spikes landed on and give out 1d4 damage per spike.
Player character receives no damage from the snakes, because they're all dead. It seems that they are food meant for whatever monster it is living behind the grate at the bottom of the pit, and in 1d3 rounds it's feeding time! Consult your favourite beastiary or monster manual for a suitable beast-in-waiting.
But wait, there's a bonus: after 1d4 rounds, the snakes come back to life as undead snakes.