The last time we went to St. Maarten we did the America's Cup Racing Yachts, that was a blast. A bunch of tourists sailing racing yachts. I always recommend taking the ferry to St. John from St. Thomas and then taking a taxi to Trunk Bay. If you've not been to St. Thomas before, then it is worth staying there and visiting Magen's Bay or Coki Beach and doing some shopping. However, Trunk Bay is one of the top 10 beaches in the world. There is a self-guided snorkel trail about 50 yards from the beach, the beach is white sugar sand with shade available, and the water is warm and crystal clear (barring a storm). They have lockers, a snack bar, a place to rent snorkel equipment, and a couple of shops (shop in St. Thomas). Bring your camera for some beautiful pictures, not only at Trunk Bay but on the ride to/from Trunk Bay. Princess doesn't dock in downtown Charolette Amalie so it will cost you $4/pp to ride into town. You can catch a ferry in Charolette, but the schedule isn't every hour like it is from Red Hook. The ferry from Red Hook is $6/pp, and even though we were just there in December, I can't remember how much the park entrance to Trunk Bay. I think it was $4/pp. The cab fare to Magen's and Coki was around $10/pp each way. Another fun thing in St. Thomas is to do a catamaran sail/snorkel. They usually go to Buck Island and the snorkeling is good. I'm not impressed with the snorkeling in St. Maarten, neither was I impressed with Orient Beach, but many people love Orient.
Hope I've given you a couple of ideas. I'm sure there will be lots of other suggestions. Best wishes and have a wonderful cruise!