Chair aerobics! There's a good video on youtube you could try. You'd have to only use one leg, or neither if it's jolting the bad one, but the arms, stomach and back will get a good workout.
I pulled my Achillies tendon about 2 years ago and i couldnt do much exercise. I still cant run on it for more than about half an hour and then the pain kicks in. Its so annoying because i used to run alot. I had physiotherapy to help with my tendon, have you been given any exercises to do for your ankle and the rest of yor body. You could try speaking to a physio and enquire about what you exercises you can do. Armchair aerobics sounds like a good way to go, lol.