I'm on the same ship and itinerary as you but leaving Feb 21st 
I've been on 2 other carnival cruises (Triumph, Miracle) and can't wait until this one.
From what I've read, Puerto Rico has alot to offer if you just stroll around the shops, etc around the pier. One of the forts is within walking distance also (maybe 15 min). The reviews I read about touring the Bacardi Museum say save your money. Considering it is free to get in, charging $77 a head seems a bit much. If you are into kayaking the Bio Bay (bioluminesence bay) tour sounds absolutely amazing. I so want to go on it, but I'm travelling with my 60 year old mother with poor knees and hips.......so I won't be. Search the web on this......I found lots of sites.
St. Thomas we are doing the Coral World and Magen's Bay excursion. Magen's bay is listed as one of the top 10 beaches in the world.
St. Maarten's I've been to on another cruise itinerary. There are lots of shops right at the port, as well as a water taxi to take you downtown to shop. Taxi's are plentiful when you get off the ship so if you want to go to a beach you could catch one of those. Last year I did the Divi LIttle Bay Excursion through carnival, but I don't see it listed anymore. Orient Beach is always popular although its beach is shared with a nudist resort so you may see alot more than you ever wanted to (of all ages, sizes etc). There are some nice snorkel or scuba excursions through carnival I believe.
Not sure if I've helped. My best advice is to do independent research on the internet of each location and find out what the popular places and popular excursions are. You can usually get cheaper rates through booking privately than if you go through carnival. Only downside, leave lots of time to get back to the ship because it won't wait for you. If a Carnival excursion comes back late the ship is held. The same doesn't apply to private excursions.
I know you'll have a great time!! St. Maartents is beautiful
