In St. Maarten there is a trip out to Orient beach if you want to do a day at the beach. Lots of water activities, lounge chairs and restaurants, very popular for the cruise peoples. Next door is a nudist beach, and this one is topless, so take that into consideration. Our last trip there we booked the Rhino boats and had a ball, something different and a look at parts of the island we never saw before.
St. Thomas you can take a cab to Meagans bay or go over to St. Johns. If you have the time I'd suggest going to St. Johns/trunk bay, good snorkeling and a beautiful island, but you'll need to take the ferry so you'll need some extra time. We tried the BoB excursion this last trip, great fun and another different kind of excursion.
Barbados there is Harrison's caves and the boatyard beach, easily you can do both.
All these islands offer all kinds of activities, you'll just have to figure out what you want to do and anything you really want to do, definitely pre-book online, some of them fill up quickly.