Before the PhD took over my life, I used to volunteer at a community radio station, presenting a show. Which was great fun, and I got to be creative in a different area of life, and listen to music I love. No time for that anymore though...but as others have suggested, exercise is good too. If I had more time I'd like to volunteer at an animal shelter.
I also go to the gym as well as play tennis - very nice running about in the sunshine with others...