Nasty, nasty tricks.
My favourite involves taking a print-screen capture of their desktop then moving their icons off screen, and then replacing the existing wallpaper with the new screen capture.
Watch in delight as they repeatedly click on icons that don't do anything.
I did that to someone and it drove them nuts.
At a previous job one guy was waiting for a new place that opened to start posting jobs in his field. So I downloaded a webpage of a job posting and edited all the info to make it look like the job he was looking for. I printed the page leaving it on the printer for him to find. He must have called and bugged them for a week claiming they had posted the job opening and he wanted to apply for it.
A long time ago I worked at a drugstore. Everyone thought the manager was a jerk so occasionally the assistant manager or I would swap out the hard boiled eggs in his lunch for raw ones because he would always crack them open on his knee. It was a good way to get him out of the store for part of the day. He never did figure out you could spin them to make sure they had been cooked.
In highschool we occasionally raided the computer lab for “computer dots” (the dots punched out of the paper tapes and cards our programs were saved on). Once dumped in a car you will never get all of them out.