I call this my "footless cardio." I actually am blessed with two working feet, but sometimes they hurt. So I set up a circuit of moves that don't aggravate my PF. It alternates between standing and floor calisthenics. It's a ten minute circuit you can repeat as needed. I hope it's helpful to somebody. Here is a link:
http://www.online-stopwatch.com/full.../?c=92cfbdbiif The timer is set for one minute per exercise, then you can start it again repeat as many circuits as you like.
Some of the moves sound weird -- the "moonflower" is sort of a plie and reach, from Yogafit, the "standing swan" is from Ellen Barrett, and the mountain climbers are the low impact version. You could add light weights to make the standing moves more challenging.