A rotating wall sweeps a player to a hidden, randomized mini-dungeon. (This may force the party to split, so either treat this as a series of encounters rather than a dungeon by actively discouraging exploration, and driving the player through the space to rejoin the party, or else alternate between party groups between each mini-dungeon room.)
Roll 1d4 to determine the number of rooms.
Roll 1d4 to determine the number of monsters in each room (low-level monsters are advisable unless the entire party has been swept into the mini-dungeon).
1d6 determines the gold pieces found in each room.