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.
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