What present should I get for a 15 year-old boy?

I'm at high school and my cl is doing a "Secret Santa" game, where you pull a name out of a hat and then have to buy a present for that person. I'm a girl and somehow I ended up with a guy's name and I have NO idea what to get him. What do boys this age even like? I don't want to get him an encyclopedia or a book, although he seems pretty smart, because they're kind of lame presents and plys, they're pretty expensive. I don't want to spend more than $10 on his present, but I have no idea what to get him. By the way, I live in Romania so I don't have lots of options... Unless I get something from E-Bay but even then, it has to arrive until the 20th. I was thinking of getting him this alarm clock in the shape of a cube, it changes its' color every hour. But I'm not so sure. This guy also likes old rock bands, like Guns'n'Roses and other bands like that, I'm not really into them, and I'm worried that if I get him a t-shirt with one he'll already have. Plus, I can't really ask him what he wants because he's not supposed to know who's getting him the present. My Mom bought this cup a couple days ago but come on, how can I give a little red tea cup with little snowmen and Santa Clauses on it to a guy? Suggestions, please?