Hate to be one of the evil ones but like MemorY said, it's in good spirits... just type in computer pranks in a search engines and I guarantee you will find something that satisfies your craving... and for more fun, start chatting with him, ask odd questions, sometimes that will scare people away better than a prank. Have fun with him, don't do the usual things, try making it funny.

- Cheers,
jaguar291

Post what happens, what to know, sounds like an interesting scenario.