Place salt in the sugar container and then put sugar in the salt container.
Take every battery out of every electronic device. Replace with tootsie rolls so they aren't too upset when they find out
replace all of their family photos with pictures of giraffes.
plant carrots in a friends garden and return regularly in the night to water them.
taking a dump in their toilet.. and not flushing
Place a positive pregnancy test on their bedside table.A friend of mine told me once he would leave a zucchini on his mass transit bus seat. I don't know why. I was too dumbfounded to find out. Now I think it's kind of hilarious.