Via cruising, I've been to both St. Thomas and St. Maarten. As I'm sure you know, St. Maarten is split between Dutch and French rule. My wife and I spent the early part of our time there on the Dutch side on a sailboat racing excursion. When that was over, we took a taxi to the French side and spent some time at one of the "clothing optional" beaches there. We also did some jet skiing just off the beach while there.
We didn't really see much in the way of shopping where we visited, and out trek across the island via taxi wasn't awe-inspiring, but we had a great time.
On that same cruise, we also visited St. Thomas but we also had the opportunity to spend the morning at the neighboring island of St. John (on the advice of our cruise director). I highly recommend St. John, because it is much more secluded than any other island I've been to, and it's just a short ferry ride from St. Thomas. We found a fantastic semi-private beach there and absolutely loved it.
We went to Jamaica, Grand Cayman, and Cozumel last summer on a cruise as well. All the advice above on these islands is spot-on. If you're looking for seclusion and privacy, I would definitely avoid Cozumel. Grand Cayman had the best water, but I wasn't entirely impressed with the landscape of the island itself. Jamaica's landscape, however, was thoroughly beautiful.
Whatever you decide, just know that you cannot go wrong in the Caribbean