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.
I picked up a cheap acoustic u-bass during the Covid years just to be able to play along with old songs and some soothing melodic solo bass laying on my couch. They are pretty quiet, enough for solo playing without disturbing the neighbors, but the vibrations from the instrument body travel really nice on top the belly. Also it sounds nice and boingy like an upright bass. Perfect for faux jazz and blues.