Ok, I was there a year and a half ago. If I remember correctly, there are a number of tours that offer sightseeing tours of St. Thomas. I did the Views of St. Thomas tour. I really enjoyed Mountain Top and the Paradise Point Tram - the rest of the tour I could have done without. If you are interested in that, take taxi up to Mountain Top (there is the best Banana Daquiri up at the bar up there) and walk across the street from Havensight Mall (just as good shopping there as downtown and not nearly the crowds) and take the tram up to the top and get a great view of the harbor and Charlotte Amalie. There are a number of excursions that include snorkeling (either the Wild Thing or the excursions to St. Johns.) You could also go to Coki Beach and do some snorkeling (check out <http://www.cokidive.com> ) that you could arrange on your own.
As for St. Maarten, I recommend two different excursions - the first one is the Golden Eagle Outisland sail - a fantastic ride on a catamaran to a beach on a small island near S. Maarten with free food and drinks there and back with a crew that is really good. The other is an excursion that my friend did and that was the America's Cup Regatta - you actually crew yachts in a race - you can do anything from watch or keep time to actually hoist sails, etc.