Piano! The apartment I moved into fresh outta college just so happened to have an older upright. The thing is terribly out of tune, a few of the keys stick, and the action is shot to hell but I love the thing so much. Any day, no matter how rough work is or how exhausted I feel, I can always find time for a song or two. I'm so lucky to have gotten into piano when I was a kid. Being able to carry something like that around with me - I'm planning on getting an electric keyboard later this year for when I end up moving again - is so immensely satisfying. And on the nights I'm too tired to play, having playlist after playlist of upbeat music (mostly hard bop and an irresponsible amount of Yoko Kanno) brings me down and right back up again. Sorry if the answer seems a bit out of sorts but I find that my "winding down" usually involves a lot of "winding up" on the tail end. I've got things like RainyMood when I need to seriously get to sleep or completely power down, but the best stress relief and mental rewiring I've found is equal parts relaxation and excitation.
I can so relate to this! I play guitar to unwind and, if I'm having a particularly stressful day, sometimes I'll start playing scales and suddenly realized that it's been an hour or two. Lately, I've been playing classical more than anything else, and it's really so lovely. Music is wonderful, and letting your brain focus on something creative feels really great.