I think Nassau has para sailing now, I'm going on the same cruise itinerary as you are next winter. My plans are private tour on Antigua of the island and snorkel with sting rays (12 year old should love that, message me if you want name of guide) in Tortola I'm going to Virgin Gorda to see the world famous Baths beach, in St.Thomas I'm going to snorkel St. John Trunk bay both the Bath's and Trunk bay are the most photographed beautiful beaches! In Nassau I think I will spend the day at Blue Lagoon island as I've done most of the other things there.
I love also love the zoo there called Ardastra Gardens and Zoo where one can feed the parakeets by hand, and watch the trained flamingos march. St Thomas I have been to Sapphire beach and did a private tour that included time to swim and shop as well as the mountain top several companies do this and you can save a lot of money by not booking with the cruise line. Also google Coral World St. Thomas lot's to do and see there from sea helmet dive, to snuba etc....
Carnival Mardi Gras 1987 (Bahamas)
Carnival Legend 2005 (Exotic Western Caribbean)
Costa Mediterrenia 2005 (Eastern Caribbean)
Carnival Pride October 2006 (Mexican Rivera)
Carnival Legend 2007 (Western Caribbean)
Carnival Glory 2009 Western Caribbean
Carnival Liberty 2011 Eastern Caribbean
Norwegian Spirit 2011 Western Caribbean Carnival Miracle 2012 Southern Caribbean
In Antigua Carnival has an excursion called "See Antigua by Sea". At the pier you board a huge Cat called "Excellence". It circles the entire island with points of interest described along the way. Thers is a long stop for swimming, snorkling, and lunch. Drinks and lunch are included. Excursion is about 6.5 hrs in length. Have done it several times and highly recommend it. Seems like cost is around $100 pp.