That kind of depends on what you like to do. My wife and I really enjoy snorkeling and the beach, so we always book at least two of these trips. If you just want to go to a beach, I recommend simply taking a taxi when you get off the ship, that way you can enjoy as much time as you like without having to leave when your tour leaves.
We have taken the same trip/itinerary two times now and really loved it.
In Aruba, take your pick of the Catamaran snorkeling tours...the Pelican Adventures company is well run and their excursions are great.
I have been to Kontiki Beach Club in Curacao and it is a really nice place to spend the day. Simply take a taxi from town. It is about a 10-15 minute ride.
St. Maarten. Beautiful Island. Again, we have taken the catamaran snorkeling tours and found them to be quite enjoyable. Orient Bay Beach is the most popular beach on the Island, but it is certainly not the only good one. We have been to Pinel Island and Mullet Bay Beach and enjoyed both of them. We went to each of them on different snorkeling tours.
St. Thomas. On our first trip, we took a taxi to Magen's Bay Beach and loved it. It is beautiful. On our most recent trip (last month) we took the ferry to St. John and then taxied to Trunk Bay Beach. It is truly one of the most beautiful places on earth. If you do this, just make sure that you ask to take the ferry from the pier terminal and not from Redhook. This will save you time.
Good luck and feel free to ask if you have any more questions about this ship or itinerary.