There is lots to do in Nassau, especially if you like the beach. You guys will have a great time on the Mariner. She is an excellent ship with tons to do even without getting off the ship. Check out www.CaribbeanPortReviews.com for details of what to do in Nassau.
As Tom stated there is lots to do in Nassau. One of my favorites is to take a catamaran snorkel trip to Rose Island. If this is your first visit to Nassau, make sure to grab a taxi and check out Atlantis and Paradise Island. The water park/pool area is reserved for hotel guests, but you can still see a lot. Downtown Nassau and Paradise Island are total opposites in appearance and atmosphere.
Nancy