Somewhere between yoga and martial arts is Tai Chi. My mom tried it and liked it, but she was just watching videos and it didn't stick. Does some muscle toning and helps with core / balance a lot.
They also have those water classes where they use pool noodles and such for weight resistance. Maybe doing that with a little swimming would be a good low-impact way to build some muscle, esp if you like swimming anyway.
Water walking/movement imo is really underrated for all levels of fitness/age etc.