Here is another one a good friend has pulled at weddings ...twice.
Buy a real nice "designer" bath towel set. Then go buy, and empty, about five spray cans of scotchguard into them. Wrap them up nice in gift wrap and give as a wedding gift. Once they realize things like "drying off in the shower is only moving the water from one side of thier body to the other", and handtowels under the sink with water running is "simply sliding off", (actual quotes from the prank-ees) you can then take them back, have them professionally cleaned and then return them (or wash them yourself about 10 times or so and they will be fine). No harm, no foul. ;-)