I already play clarinet, as well as my chords of vocals, but I would really like to learn how to play guitar. If I couldn't learn that, I'd like to learn piano or the bells, but I've heard they're very similar.
I play the Classical Guitar, it's a pretty fun instrument and it can make very clean and smooth sounds. But the downside is that it takes a very long time to get good at.
I play the violin and have for 15 years. If I was going to play another instrument I think I'd go with the guitar or bass guitar, if I didn't go the traditional route and take up the viola.
I'm lucky and have a teacher who'd be willing to teach me the guitar and bass guitar. I used to play guitar, but quit to focus more on the violin. The bass guitar I dabbled with but it didn't go far.