Exercising in a cast

I've been doing great since starting at MFP in November, but I'm concerned that my progress will stall this spring. Sometime April-ish, I am planning on have a foot surgery that will put me in a cast for 2 months. I will be largely home-bound. I am concerned about switching to primarily upper-body exercises, since I know from last time I was on crutches that they can be tough on the arms/pits.

Any suggestions for aerobic exercises I can do in cast?

Thanks