A really fun prank to pull is to go on your SO's computer, take a screen shot of their desktop back ground, including the desktop icons, then replace their desktop background with the screen shot you just took. Right click on the desktop, go to the VIEW tab and then uncheck the SHOW DESKTOP ICONS. When the person goes on their computer the next time and attempts to click on any of the icons nothing will happen. Click... click click... CLICK CLICK CLICK!!!!! NOTHING WORKS!!!!
Easy to put back the way it was and causes no long lasting damage to the 'victim' or the computer. It's great fun to see them clicking away and not being able to understand why their computer is not opening Firefox or IMVU or IM or whatever program it may be that they're trying to click on. Best part is when they realize one icon doesn't work they will start clicking on all the other icons frantically which just adds to the hilarity of the situation.