As the proud survivor of 13 ankle sprains and two broken ankles I certainly feel your pain. As to treating and cardio, it will somewhat be determined by the type of sprain. High ankle sprains are a bitch and can give you problems for a while. In the first couple of days after the sprain, it is best to keep your foot elevated and use as much ice as you can stand to get the swelling down. After a few days, if your sprain is severe, your foot will turn black. At this point it helps to alternate hot and cold treatments. Cardio is tough. If you happen to know someone who works in a PT office you can use their hand bike (used to treat arm and shoulder injuries). You can get a little bit of a workout from that. You could try swimming, but if you try and kick you could easily aggrivate the injury. The best thing that I have found is an air cast. You should have a prosthetics maker in your area and they stock these things. They can be worn under a tennis shoe and would probably be stiff enough to ride a stationary bike. I would stay away from recumbants because of the angle.