It's not exercise (outside the hiking) but ... camping is amazing and a great start if you're not inclined to do much. Limited responsibilities, and the responsibilities you do have (cooking, staying warm, not dying) help sort of reset some biological imperative that gets lost when you're entirely too comfortable at home on the couch.

Any form of cycling really clears my head. The sensory stimulation, the forced focus, the sense of freedom, it's wonderful.