Maybe some gentle yoga style moves or even ti chi?? - you can find samples or types on you tube & Google videos - for beginners, seniors or arthritis would be the easiest to try.

I read somewhere that swimming is good for fluidity & movement -and if you can't bear weight on your legs or joints, but because the water buoys you up some of the weight bearing & muscle fitness benefits are not there. So you'd have to do more swimming to get equal benefits.