From Trip Advisor:
My adult daughter is a walker/wheelchair user and it was great to see that Castaway Cay truly is fully accessible, as we had been told. We used our own wheelchair to exit the ship and were able to walk all over the island on the paved paths. When we wanted to go directly on to the sand, there are multiple highly visible spots all over the island where you can pick up a sand wheelchair. We just left our wheelchair with the attendant, exchanged it for a sand wheelchair, had fun on the beach, then got our own wheelchair back. You can move around from beach to beach this way if you want to - use your own chair on the paved paths to get from one beach to the next, then when you get to the beach, turn in your chair for a sand chair - it was much easier and more straightforward than I had thought it would be. Like everything else, Disney does it right.
We very much enjoyed the adults-only beach, Serenity Bay. There is a bar there and a BBQ - only for those over the age of 18 - so if you are traveling without children, you can go straight there and spend the entire day. We even had a cabana massage there, so idyllic, in a cabana directly on the beach, the sound of the waves and the palms surrounding you, lovely!