How are you supposed to exercise when your wrist is in a cast?

Silly question time.

I have a cast on my left wrist from crashing my bike. It looks a lot like this:

I shouldn't get it wet, and I have to shower with it in some comical arm-condom contraption from Limbo.

I also miss exercise massively, but the idea of exercising and therefore sweating into a cast that can't wash for weeks doesn't exactly sound attractive.

What are you supposed to do if you want to stay active and retain some fitness over the 6+ weeks you're in plaster?