martial arts was so far the best thing i did. it has so many positive aspects that there's just too many to name. but an overlooked one worth mentioning is that it is also a social activity and programmers could use a lot more of that.
unfortunately i have no gym i can go to nowadays.
i started seriously running this year and i like it.
going for a motorcycle ride is also nice, especially if you have good roads near by(which I don't).
going for a long walk with your dog, somewhere into the nature.
personally, when i find something new that interests me, i usually obsess to achieve 90% mastery/knowledge and the move on. it can be absolutely anything. inspiration can come out of nowhere. so nothing specific to recommend as it is always personal.
buy a telescope and go skywatching :)
ps: i bought an electric guitar that i wanted to lean but it is quite hard and i am not obsessing over it so it stays in the cabinet :D
