If they have a laptop/pc with windows and you can get access to it for a few seconds, try Ctrl+Alt+down arrow which flips the screen so it's upside down. When they come back they will be like wtf? But make sure you use Ctrl+Alt+up arrow to get it back to normal afterwards.
I've done this a few times and others have followed. Happy Birthday. Call their friends to send cards and E-Mails and leave voice messages wishing a happy birthday. Leave little gifts on their desk at work. It's harmless and drives them crazy all day long.