we used to have a game at the last place I worked...
if anyone left their machine unlocked then it was open season...
putting their desktop as a background and hidding all their icons is a timeless classic...
pressing shift five times is sure to annoy anyone,
swap mouse buttons...
hide documents.
turn on disability contrast options...
insert curse words into peoples work...
Now, I'm into a firm habbit of hitting windowkey+L each time I stand up... (and locking my office door!)