Swimming is excellent because it's as close to zero-impact (e.g. not pounding pavement) as you can get. There was a point in my work career where I hurt and swimming every day after work helped.

If you're wondering or thinking "Swimming is hard" - go to http://totalimmersion.net (they teach you how to swim like fish do - not the standard Red Cross Mississippi Steamboat (paddle-wheel high-energy-required kick way)