Take a screenshot of his desktop, move all of the icons off it, then set the screen shot as his wallpaper.
Plugging in an extra mouse is a good one.
All those have been done to death here. Also wireless keyboard instead of mouse. rotating desktops, faked virus alers, lots of rick rolling type stuff "HEY YOU'RE LOOKING AT PORN", etc. Hiding their stuff. Anything you did in primary school works well.
We did a lot of 'sleepers' like opening up computers and leaving pints of milk in them. Taping "I love Westham" on the side of the Chelsea fan's pc.
All those have been done to death here. Also wireless keyboard instead of mouse. rotating desktops, faked virus alers, lots of rick rolling type stuff "HEY YOU'RE LOOKING AT PORN", etc. Hiding their stuff. Anything you did in primary school works well.
We did a lot of 'sleepers' like opening up computers and leaving pints of milk in them. Taping "I love Westham" on the side of the Chelsea fan's pc.