A pressure plate causes itself as well as every second tile in the room (in the pattern of a chessboard) to quickly elevate, lifting anyone standing on one 20 feet into the air.
This presents the players with at least 3 options:
If a creature on the pressure plate jumps down, taking falling damage, then all other tiles lower gently.
If everyone else on a raised tile jumps down, taking falling damage, then the pressure plate lowers gently.
If everyone jumps down at the same time, then the remaining tiles raise quickly to break their fall such that no one takes falling damage.
If option 3 was taken, a door at the level of the new "floor" may be revealed as needed.