My prank is called the Hellburger.
The premise is this. Convince a castmember (the victim) that he has to compete against another dude in eating a burger, whoever eats it the fastest wins a random prize (money, date with a chick, etc). Really hype the thing up though. What the victim won't know is...his burger will be spiked. I think hot peppers is the way to go here, like Habanero peppers or Atomic hot sauce or both!! Then, make sure there's no water, soda, beer, any kind of relief in sight, turn off the water faucets if necessary. It would be extra funny if the victim can't tolerate hot food, and when he's twitching on the floor in agony, someone kicks him in the peepee.