Sorry in advance for the longish post.
Several years ago, I worked in an ESL training centre in China, with an American guy called Jacob. He was going a bit stir-crazy living in China, so I started fucking with him at work to make both of our dull jobs more interesting. It went on for about 9 months, he was a good sport about it.
There were two pranks I was particularly proud of.
First one. I took a whiteboard marker one day, and wrote 'Jacob' in mirror writing on the office photocopier glass. It took about an hour for it to get discovered and rubbed off and during this time several people had photocopied stuff, then discovered the name Jacob watermarking their documents. Of course I re-wrote 'Jacob' again as soon as the coast was clear.
After about a day of this people started checking the glass for the Jacob, and at this point I took out a ream of paper, ran all of it thorough for a Jacob print, then put it back into the tray. This caused a lot of confusion. Jacob was well aware of what was happening and of course knew it was me (when something happened it was always me), but he didn't catch me in the act.
The day after this we had a staff meeting, and I took the opportunity to point out to our manager that someone had been writing Jacob on the photocopier glass. I went on to advance my theory that it may have been Jacob who was doing it. He spluttered out a mouthful of the tea we were all drinking, I like to think it was in equal parts of frustration and admiration.
The second one; he commented a lot about the lack of Dunkin' Donuts in China, he was particularly missing their coffee. Well, my wife and I took a short vacation in Malaysia, and I made sure to stop off in Dunkin' Donuts and collect a paper doughnut bag. On my first morning back in work I bought a cheap doughnut and put it in the bag, then took it out and ate it in front of him. He was happily in shock for a full 30 seconds before he realized I was probably just fucking with him. I actually felt bad after this - he was genuinely disappointed. He went back to the USA soon afterwards.
I would just like to state again, I pulled these pranks because I genuinely liked his company and wanted to make his life more interesting. And for full disclosure, I'm from Europe and I don't 'donut'.