Exercises you can do with Crutches.
I recently had a Plantar Fasciotomy/Release and I have been non-weight bearing for 3 weeks. I was going crazy, so I had to start doing some kind of exercise. I did a video of some of the stuff I have been doing. What kind of creative exercises would you do? Check out the video and suggest what else you would do to get the heart rate up without using my other leg? Here is the Video: http://www.youtube.com/watch?v=FdH_giemmcg
One of my first clients was on crutches just as we were beginning to exercise (I had no part in it). One thing I learned from that is that some people very quickly can develop shoulder and elbow problems. Ever since then I advise people who find themselves on crutches to work on rotator cuff exercises, scapular depression and wrist flexion and extension.
Abdominal muscles are just like any other muscle. Doing hundreds of sit ups does not engage a core any more than basic balance exercises.
Treat them as any other muscle. Weight bearing, 3 sets 8-20 reps.
If people are looking to "get abs" tell them they're made in the kitchen, not the gym.