How about swimming or aqua-aerobics?
If you belong to a gym, check out the recumbant stationary bikes as well - as a matter of fact you might consider trying out a recumbant road bike - when hubby and I were working for a bike shop we sold quite a few recumbants made by BikeE -- here's their website
http://www.ihpva.org/com/BikeE/BikeE.brochure.html
I would also strongly recommend (with your doctor's approval of course) including some strength training exercises to get those weak areas stronger - again, do discuss with your physician!
OH and here's an article I found:
Quote:
Recumbent Bicycles - A Safe Laid - Back Approach
Hop on a recumbent bike and you feel like you are sitting in a couch. Not quite - but close. It is comfortable, but there is work involved. These laid back bikes are found in health clubs, rehabilitation centers, offices and homes. They are lower to the ground and you sit in a upright position in a large seat with a generous back support and your legs are stretched horizontally. As a fitness consultant advising clients on purchasing equipment for thier home or office, it is the first choice I recommend. They have been around since the early 1800's! Besides offering a good variety from other types of cardio machines, here are some advantages:
Greater comfort and back support. If you are an individual that suffers from back, neck, shoulder, knee or any type of problem, this bike is more comfortable than the upright bikes because it takes the pressure off your bones surrounding your gluteus maximus (your buttocks), and distributes the weight evenly from the back to the legs.
The muscles of the lower body are worked differently. The buttocks and hamstrings are worked more than upright cycling along with your quadriceps (front thigh) and your calf muscles.
Like any cardio machine, it benefits your heart. The aerobic benefits are similar if not more, to any cardio machine on the market.
A great machine for the unfit. Because your legs are almost at the level of your heart, your blood pressure and heart rate will not rise as fast as an upright bike, thus letting you ride farther at the same workload. If you have any physical disabilities these bikes are easier to climb on and off.
Words of advice for the buyer :
Recumbent bikes are found in exercise stores and health clubs. Test different models before investing.
Make sure the model is built by a reputable company that offers at least a 90 day warranty.
Think of your bike like a new car that will give you years of enjoyment with proper usage.
Remember that when first starting or changing an exercise program, you should see a physician to have a fitness check-up and have your medical history evaluated for potential risk factors. Happy riding!........
I've tried walking, but by the time I put in 15-20 minutes my back and ankles are killing me. Have any of you had the same problem? If so, what types of exercises have you been able to do? Thanks in advance for your responses!!! 
