If the skin on your ankle is fully healed and you can submerse it, you could try swimming "pulls" (i.e. using only your upper body, no kicking) in a lap pool.
If you can put some weight on the ankle, try inclined pushups/pullups. Pick an incline that will reduce the effort and go for lots and lots of reps. Unless you're in really top shape, regular pushups and chinups will not be aerobic.
How about juggling?
Wikipedia tells us
As exercise, juggling is a highly aerobic activity, increasing the heart rate and respiration. Juggling helps one to develop good hand-eye coordination, physical fitness and balance.
That's just off the top of my head. But now that I read that wiki entry I think I am going to learn how to juggle!
