Weight bearing exercises are good, to strengthen the joints. Walking on a tradmill / walking uphill on a treadmill is good. Cycling on low resistance (if you're using one in a gym), then gradually increasing it will help in the strengthening. But you would have to speak to your doctor to get refferal letter first. If you can find a gym that has a hand bike, then that would be overall good aerobic exercise. A benefit for this machine is you'd be getting a good workout and you dont use your feet for this either, so no pressure would be put on the ankles. You may want to ask your doctor about exercise on prescription. A doctor refers you to a gym that has a health advisor. Who will work out a program for the gym, monitor your progress and help you with exercise outside of the gym. As for fat loss it's to do with diet as well as exercise. Any gym that has a nutritionalist will be of benefit to you.