For girls, (young or highschool age girls) the best prank is to go up to your mom or dad, and just as straight faced as you can, tell them you're pregnant. :lol:
Another good one is a packet of ketchup or even hot sauce under the little feet on the toilet seat.
You can also take the valve core out of the air valves on someones wheel, and very quickly put the cap back on. It will usually hold until they start to drive off, and then it will almost immediately pop off and let all the air out. Then when they re-fill the tire, it all comes back out as soon as they take the air hose off.