Here's an excellent prank to play in a computer lab on a computer-illiterate person.
First, make sure your victim is using a word processor. Second, make sure your victim isn't the type of person that complains about every little thing that goes wrong. Third. Make sure your computer is right in front of your victim's computer. Make sure you can unplug the keyboard.
Wait until your victim leaves the room. While they are gone, pull out their keyboard and plug yours in. Type something like, "Welcome back..."
See if you can start up a 'conversation' with your victim. Try to get them to talk to the computer, and type replies.
Just make sure you don't get caught.