Black On Black
Change the prompt on someones computer to be black on black. This is rather cruel if the person is computer illiterate. Very effective the day before a big project is due.

Magnet cup
Glue a magnet to the bottom of a cup and stick it on the roof of your car and drive away. When they try to get your attention take a sharp corner. Their look on their face is priceless.

Bar Of Soap Lather Prank
Take some nail polish and coat a bar of soap with it. Let it dry. Then put it in the bathroom shower. When your victim tries to use it, he or she will go nuts trying to get it to lather up.

Unable To Open
Glue all the bottles of shampoo shut so they cannot be opened.

Pop!
Put a balloon over the hole of the victim's exhaust pipe. After he drives away...a few blocks later, he will hear a big POP!!