I ski (although I skipped this past winter and focused on indoor tennis). I also run a fair bit (participate in the occasional race) and do some outdoors stuff like hiking, cycling and rock climbing. I also lift regularly at the gym.
My latest thing though has been yoga. I'm really bad about remembering to stretch (and actually I hate it). But I've been doing yoga for about a year and a half and love it. I can also feel it helping me in all my other activities, because I just feel looser in general and my muscles feel more relaxed. Helps me extend fully on my serve and groundies.
I was thinking of starting fencing. I've taken some lessons in the past, but didn't have time to stick with it. It actually requires a lot of the same skills as tennis--hand-eye coordination; strong quads and ham strings to keep the "knees bent" stance; and strong footwork.