(In no particular order)
1. You’re good looking for someone in a chair.
2. I had to use a wheelchair when I broke my leg, so I know exactly what you are going through.
3. You're an inspiration.
4. I was only parked there for a minute.
5. Everything happens for a reason.
6. Here, let me help you.
7. It's good to see you out.
8. Have you got any more movement?
9. With your effort and attitude, I know you will walk again.10. Anything with a kindergarten teacher voice or pat on the head.
I found most of these online but added and subtracted some that fit to my experience.