Personally I find any activity which forces you to lose that constant mind chatter to be very relaxing. My favourites are sparring (Brazilian Jiu Jitsu), weight lifting and Life Drawing. I guess it's that old 'flow' thing.

The nice thing about exercise is it will help you sleep better, which I find is half the battle in trying to maintain focus (especially for us programmers who spend most of the day sitting at a desk).