Raiders of the Lost Ark Pedestal Trap
This classic trap was made famous by Raiders of the Lost Ark. You put a valuable treasure on a pedestal - but when the pedestal senses the weight is gone, it triggers a trap.
Having a gigantic rolling ball is probably overkill. You can simply have the trap seal shut the door of the room, trapping the adventurer within. You might have the trap set off a poison gas or drop a cage down around the pedestal.
If the character does indeed suspect something and try to do the swap-one-object-for-the-other routine to keep a weight on the pedestal, you should give them a reasonable chance of success. Roll against their dexterity to see if they make the swap fast enough, so the trap mechanism is not activated.
Role Playing Monsters and Traps
Role Playing Tips and Information Main Page