How to exercise when injured....

I sprained my ankle and am on crutches for at least 2 more weeks...it has already been two weeks of being in bed and I am going insane. If my foot isn't elevated for too long, it starts to hurt so I can't be up for top long. Does anyone have any suggestions on what I can do to sort of stay in shape while I'm stuck here? The only ideas I have are maybe crunches and pushups (on my knees, of course). I just don't want to be set back too much whenever I can get back to the gym... Thank you in advance!