There are two instruments that I would like to learn.
Drums: so I can take out some of the anger and frustration and make it into a melody.
Bass guitar: I have always liked how they sound and when I played any Guitar Hero or similar games, I always played the bass. I could imagine laying in bed and playing a melancholic tune until the sadness goes away. Or just play whatever fits my mood at the moment.
I have also thought that violins look and sound interesting, but they seem pretty complicated to play.