At Nassau, I enjoyed the Atlantis Beach Day. It's a beautiful resort and I have some very striking pictures from the day. But I wouldn't pay that much money for it again. I wouldn't be able to recommend it just because of the $$$, despite how beautiful it was.
The Harbour Cruise and the Historic Tour worked well for my Dad. He got to see "the highlights" but didn't have to do alot of walking. Just keep in mind that the narration during these tours can be difficult to understand because of unfamilar accents and poor quality PA systems.
The one excursion that I would absolutely say DO NOT do is the glass bottom boat. I took my Dad on this and we were both very disappointed. The "glass" for us to look through was not very clear, and the space was cramped. There was no way for everyone on the boat to look at the fish at the same time.
I'm not exactly a "water-baby" so I haven't tried any of the snorkeling excursions. Other posters will have to give you their two-cents on those.
On our last cruise we enjoyed staying aboard the ship at Nassau. We enjoyed a leisurely sit-down lunch, and an almost empty pool. It was a very relaxing and enjoyable day on board. BTW, the spa usually offers incentives to book your treatments on that day.
On
Castaway Cay I whole heartedly recommend the banana boat ride and parasailing. I had a fantastic time on both!! Of course, most of my visits to CC I have simply enjoyed the beach and think that you can't go wrong with just relaxing at CC!