Hello there,
As for San Juan, I would have suggested the Bacardi Factory Tour, but I will probably not be available due to the late arrival. Other than that, I must say that San Juan has awesome shopping, especially for the women. There is a very nice shopping district very close (walking distance) fromt the dock. Recommended Stores: Del Sol--its a store that sells color changing merchandise, when your clothing hits sunlight, it changes colors. Very cool store, especially for any youngsters. Linen House--sells very nice linens for very very reasonable prices, great store for the women.
Those are two stores we visit EVERY time we go to San Juan and we have been there 4 times.
As for St. Thomas, I would definately recommened the SkyRide to Paradise Point. Its a gondolla that takes you up a mountain and it offers the BEST view. There is some shopping and a restaurant on the top of the mountain. Here is one of the pics I took from the top::
...I think this picture pretty much says it all.
I have not been to St. Marteen, but I hear its alot like St. Thomas.
As for the cruise itself, I have been on that ship. VERY NICE SHIP.
Carnival is a great cruiseline for newbies and experienced cruisers. We still continue to cruise Carnival after 6 cruises.
Hope this helps 
