As a spin off of the cups filled with water:
I did this my senior year. Fill up cups with water. Then put a piece of flat cardboard or something flat but not really allowing water to pass through. Flip the cup upside-down with the cardboard on top to minimize the amount of water that will be lost. Put the cup upside-down on the floor (note, it is sitting on teh cardboard). Slide the cardboard from underneath the cup. (Note, some water will spill, but if you are careful, it will not be enough to make a difference.) wipe up spilt water so it looks like an empty up, laying upside-down, just sitting there. Repeat a billion times until the room is filled with them.
With this prank, the *only* way to lift the cups will be to spill water all over the place. And interestingly enough, the water does not just leak out, but kind of splatter or explode out, surely soaking the person lifting it.
It's a really neat prank, and there is no way getting around getting wet or spilling water.
Another prank would be to jack up a few friends trucks and put a block of cement underneath their axle (just enough so that the tires are off of the ground, but it is not noticable). Then lower the jack. It is incredibly funny to watch your friend try to drive away and become completely befuddled as to why his vehicle isn't moving. He will think he can't get it out of neutral. Yea, its real funny. Don't do it to an old truck though, cuz maybe the reving of the engine may burst the motor or a head, and this would of course stink (but this isn't going to happen).