You can learn all sorts of stuff online (legitimate, educationally-verified stuff from universities) - ever looked at iTunes U? They have complete courses in various subjects.
How about photography? It gets you out and about for exercise in the fresh air and sun (but other weathers shouldn't be ignored for great photographs) and there's a never-ending supply of things to capture, people, pets, wildlife, water, mountains, technology, sport,... Even the same things from the same place look different at different times of year.
Is there a local drama group/theatre nearby? You don't have to go onstage, there's lots of things need doing in the background - building/painting sets, lighting, sound, prompting, selling tea/coffee/alcohol, selling programmes/raffle tickets, costumes...
Be a volunteer. How about helping at Scouts, Guides etc. (or whatever you have where you are) - you don't have to be a leader and you'll have plenty to do (trust me on this one!)