As GTF mentioned, if at all possible, a pool would be the most viable option. You could try a bike but I’d wonder if that would also aggrivate the heel. Just take a look at the legs of any competitive cyclist and you’ll notice that they work their calf muscles and, by extension, achilles tendons pretty hard. The other option if you have access to a fitness center with this type of equipment would be (possibly) a rowing machine or a hand bike. Again, I’m not sure if the rowing machine would aggrivate your heel but a hand bike, while being pure torture in my mind, is purely an upper body exercise. You could get in some aerobic work on one of those without doing anything below the waist other than stabilizing yourself.