Can you bike? I'm a great fan of bike riding (as a fan of the pro sport itself and of practicing it myself for recreational purposes) and that has been my salvation there. I once stopped biking for almost a year, due to a surgery, and my body inflated more and more quickly than Obama's public debt; I couldn't even run anymore. Once I was cleared to bike again, my shape and overall well-being improved drastically. Personally, I think it's a more enjoyable than gymns, treadmills, jogging or even running and jumping in pick-up ball games. You can do it alone, with friends, with the kids, with groups, road, mountain, climb, downhill, faster, slower. Plus, it's cheap; but if you want to make it expensive, there are lots of fancy stuff to buy. And you can discover new places and fabulous landscapes. Also, you get to know other riders, and riders are good people, smarter and more handsome than the average citizen.
You do live in Cape Cod, if I'm not mistaken, right? I've seen photos from there in a biking-related message board, it's a great region to bike, awesome biking trails from what I've seen and there are plenty of clubs and group rides. If your knees allow it, give biking a try!