One of the pranks that I know of that isn't really harmful but still amusing is the "bathtub jello" one.
I've heard about couples returning from their honeymoon to find nothing out of the ordinary - until they go to take a shower/bath, when they find that their bathtub is full of jello.
It's easy to do - although I think it's better to make in layers if you have the time (you can assure that it's set that way).
Just run hot water, dump in lots of jello and *lots* of ice cubes, then stir.
It's somewhat of a mess to get rid of, though (you have to melt it a little bit at a time or scoop most of it).Â
Just a warning about the jello though...be sure to put it in the tub no more than a day before it will be discovered.
My fiancÃ© filled the tub a week before the honeymooners got home, and it went mouldy.