[AncewwewBus]

Take screenshot of desktop, set as background, hide toolbars etc.
Changed the regional language to chinese.

Waited until my step-son had gone for lunch, popped onto his PC, and opened every naughty web link he'd visited in the last week and left them all open on his desktop, and changed his desktop wallpaper to a picture saying "Stop saying you know lots about computers cos you can't even delete your internet history :-P".

Various other stuff to cause much annoyance.