Hey Robin. First off, 51 is not old at all! My parents are avid cruisers and they are entering their 70's. They do whatever they want on when the cruise. As for Nassau, I was there years ago when I went on another family cruise (Fantasy '91). I am personally going to Atlantis because I have been wanting to see it for awhile now. My sister her kids and my parents have already been there before so I am not sure if they will go with me and my partner. i think we are just going to take a cab and buy a one day pass. That way we can do the water park and then tour the grounds and beach after it shuts down. I am thinking of doing it this way since the ship is docked from 12-10 PM. I don't want to do the Carnival Atlantis tour because it would take up too much time on the front end of the day and I am sure the water park closes around 5. Since the ship leaves port so late we can tour the grounds after the water park closes. I am assuming everything there will still be open even though it will be Christmas Day.
Aside from that there should be a few things you can do in port. You could just take the excursion to the Atlantis beach and hang out there all day. I think the beach on Paradise Island is called Cabbage Beach and I have gheard good things about it. It isn't near as much as the other tour. Atlantis is not that far from the dock so even getting a cab on your own should be pretty reasonable. Check out the destination forum on the Bahamas. There are a lot of threads with suggestions on there. I hope you have a wonderful time! Keep posting on this thread and maybe we (and some others) can help each other out. Good luck finding a cabin mate also.