My first trip to the Eastern Caribbean was on the Magic a LOOOONG time ago. This was without kids. We did the St. Maarten Island Tour. It was ok but nothing special. It was interesting to see how each side of the island is so different (Dutch vs. French). On St Thomas, we did the St John Eco Hike. I have traveled to many countries and this was by far one of my most favorite memorable beautiful places I have ever seen. You take a ferry from St Thomas to St John, meet up with a tour guide (ours was so informative), hike for an hour or two around the island, visit an old sugar mill, and end up on this private deserted beach where you have about an hour to swim and relax. It was really amazing.
Second trip was last year with an 8 year old on the Fantasy. For St Maarten, we wanted something active so we went with the downhill hike, forgot what it was called. It was really cool!! You take this open air truck to the top of a mountain and then hike all the way down. The tour guide is what made it stand out. For St Thomas, I didn't want to miss seeing St John again, so we did the Champagne Catamaran. It was wonderful!! The water was so calm. You board a catamaran from St Thomas and sail over to St John. We stopped at a beach where you could relax or snorkel. Again, St John is one of the most beautiful places I have ever seen.
Hope that helps!!!