Swap the M and N keys round …
Then turn co-worker’s drawer unit round so the back is facing them …
Then try not to laugh too hard when they realise what’s happened and go for their penknife in the drawer to lever the keys off …
Ghost mouse …
Tie a piece of string to their mouse and move it as they reach for it (don’t do this if co-worker has a weak heart).
And the universal yellow sticky on the bottom of the mouse so the laser can’t see any movement… removing ball from old-style mouse
Swap co-workers mice and keyboards around …
I don’t do this stuff any more because I upset someone too much and did’t want to do it again. But if you’re in the mood …