I always liked playing around with the backdrops, usually got a comedy one on it (not that they are usually in sight, I run twin monitors and would love a third lol)
One I always like putting up is this
https://i.postimg.cc/tJt6shym/mypornstash-desktop.png
which sits the 'folder' in a blank spot on my desktop, funny watching people trying to click it LOL
Also fun is to get their desktop icon to a commonly used program, do the same (put a image with the icon they normally click on in its usual spot) then rename the original icon and change its screen display, then move it to a different spot (right next to where it used to be is good or in a far corner) then watching them 'clicking it' repeatedly trying to open their program, then step in when they say it isnt working and discretely click on the new position while moving towards the 'icon' and obviously clicking on it
'works fine for me' as the program starts up, then close it back down and watch them clicking away at the 'icon' again..
Offer helpful hints like "Are you sure you are using the mouse right" and "Maybe click it faster?"
I have a very warped sense of humour...