Use a very small needle to poke some holes in a cup, but not too big. They'll need to not realize their drink is getting drained until it's too late!
Tape a few magnets to the bottom of an anyday household item (coffee cup, purse, drink, etc) and rig it to the top of your car, that way people driving by will be amazed!
Buy a lottery ticket and give to the unsuspecting victim the day before the lottery. After the lottey is over, buy a ticket with those exact same numbers and switch them out. They'll think they've won big!
Set the victims alarm clock to the same time it's usually set for, but do it the opposite time of day. If they have to wake up at 10 in themorning, set it for 10 at night!
Tape the hook of the victims phone down. Even if they try to answer the phone it'll continue to ring.
Take a piece of tape or paper and stick it to the bottom of your friends optical mouse. When they use it, the mouse won't move and they'll never think to check the bottom, until about 5 minutes of frustration.