Hi Karen,
Mary Ann from Maryland here! We are basically doing our own thing on the islands. We will probably shop on St. Maarten and then try to snorkle somewhere on St. Thomas. We have considered going over to St. John but have about decided not to do that. The "up" side to going on a Disney sponsored excursion is that you don't have to worry about getting "left" on shore if you are with an excursion, and you don't have to plan anything. The "down" side is that they are usually much more expensive than doing it on your own, you are in a Large group of people, and it is totally inflexible.
Looking forward to meeting you on the cruise!