There are two low impact exercises that may help.

I would walk about a mile in a lap pool. A local health club had a lap pool that I would walk/lung for about 45 minutes to an hour. It was refreshing work without the windedness from other exercises. The reduced weight from the flotation makes it so the steps do not send impacts to your head.

You can also buy a chair based bicycle pedal system. You sit in a chair and pedal to your hearts content. They are $20 to $40. Some have a counter to track how many rotations you have done. Here is an Amazon link to the one my mother has. http://www.amazon.com/Drive-Medical-...=cm_cr_pr_pb_t