<human>: As the title says, I would like to play some sort of computer prank on my roommate. I have access to the router and his network drives, which include a public folder, admin folder, and his user folder, inside of which is his desktop folder and downloads folder. I don't want to do anything that's going to damage the computer, I just want to mess around with him a little.

I first thought of messing with DNS settings in the router to redirect facebook or google to another site, but couldn't find any way of doing that without installing another firmware on the router. My other thought was to place a file on his desktop, but I couldn't think of anything good, so I've come to reddit for advice.

Give me a shortened version of the text
<bot>: What are some good, relatively harmless, pranks that I can play on my roommate without direct access to his computer?