I put a post-it note with 'April Fools' written on it and stick it under other people's mouse at their computer. Love watching people chuck a spack attack at their computer and then get down and checking their tower/usb connection, then restarting the computer while impatiently tapping the mouse! Takes a while to work it out.
Simple, yet effective. And importantly - tasteful humour for the office environment.
pinkboy