Basically you split the room/group in to two teams (I really like guys vs. girls). You then choose a random "champion" to come to the front (can be from either team, or it can be an adult leader). Tell each team you are about to give them a task to do, but you won't tell them what it is. They must "bet" on whether or not the champion can do what you're going to ask them to do. If they are right, they get a point (or a million points!). At the end of however many rounds you choose to do, the team with the most points wins.
For the tasks, come up with some silly, random stuff. Examples are:
-Do a cartwheel.
-Touch their tongue to their nose.
-Juggle three balls.
-Name all fifty states.
-Name the last four presidents.
-Name three people that you pick out of the group (first and last names).
-Eat a packet of kool-aid while keeping a straight face.
-Solve a long division problem.
-Eat 10 saltine crackers in less than 60 seconds.
Try this out - tweak the rules to fit your situation, make up some fun challenges (or better yet, have students write some down to use!), and have fun!