Hi. I agree with the other responses, that swimming or aqua aerobics is nice low impact alternative. If you have access to a gym, you may consider a great machine called the NuStep which mimics a stairmaster sitting down and involves the upper body. Also, any recumbent bike and/or airdyne bike will work as well. Strength training will significantly help with weight loss as well. Don't worry, you won't get huge, but the strength training will help boost your metabolism which will help with weight loss. Plus you can stimulate all of your muscle groups without putting pressure on your ankle. Just make sure you have your physician's approval before starting any exercise program.
So, combining aerobic exercise, strength training and a healthy diet together will spell success! If you are considering joining a gym, please consult a CERTIFIED personal trainer or fitness instructor for help - don't be afraid to ask to see their credentials too. Remember that comittment to a healthier way of life is a lifestyle change. Don't expect results overnight....it takes time, patience and persistence.
Best wishes!