Some monitors have a cable with five BNC connectors at the monitor end: black, white, red, green, and blue. It's fun to mess with these - like switching the red, green, and blue cables around for interesting colour effects: each cabled carries the individual colour components that form the actual picture on the screen. You can substitute red for green, green for blue and blue for red, if you like. Don't touch the black and white cables, though.

WARNING: Monitors are high voltage devices, and are as such potentially dangerous: always power-down and unplug your monitor first.

If your friend's computer relies on a modem it can be very funny to change the telephone number which their computer dials when connecting to the internet to that of their mobile phone!

Everytime they try to connect their mobile will ring, and when they answer it they'll just hear strange noises.

NB: you must write down and keep the original phone number so that you can put the real number back to what it should be. It is better to be in their house when they try to connect so that when they realise it's a joke you can fix it quickly.

Instead of changing the whole keyboard. Only switch the "m" and the "n" keys. This works best for people who can touch type well.