Which instrument should I play?

I'd go with drums. If you just want to play rock, drums are really pretty easy to pick up, even if you don't get any training (not to say you shouldn't, but I learned just by trying to play with songs I knew and watching guys play beats on youtube). Plus, they're lots of fun to play, and you can just play along with a song without having any sheet music or anything. You just feel the beat and go with it. Not to mention the fact that you don't actually have to be able to read pitches or play scales or anything like that (you need to know rudiments and a few different symbols, but you should be pretty good with that if you already know how to read music). I'd go with drums. Good luck!