This is something my younger sister and I find absolutely hilarious...
You go to walmart...you grab a box of condoms and you walk around the store looking for a the sad little victim that has walked away from their cart and you ditch the condoms in their cart. I don't know why it is sooo funny but every time we do it we laugh and laugh and laugh. The funny part is not when you dump off the condom it's how you think they will react to it once they find the condoms! It is one of the funniest little pranks I have done in a long time!