We didn't go on excursion in St. Maarten. Just walked 10 minutes to Philipsburg and strolled around the street. I was shocked to see all those diamond shops! Why would anyone want to buy diamonds in remote Caribbean islands is beyond me. The town itself is very touristy, with some interesting buildings however. We took water taxi back to the port afterwards. People said that Marigot is more interesting? We will try that out next time.
Initially we booked St. John snorkeling and beach break. The morning when we arrived St. Thomas, we were told the excursion was cancelled due to mechanical problems. We called Concierge and they were able to help us booking the St. Thomas tour with Magens Beach break quickly. It's a nice excursion with three parts - first you are taken by open-air safari bus to a couple of vantage points with views of gorgeous St. Thomas port and Magen's beach. Second you are taken to Magen's beach and get to spend ~2 hours there. Magen's beach is quite nice but I don't believe it's too extraordinary for Caribbean beaches. The final stop is at Black Beard's castle - it is actually a quite large complex built on the side of the slope facing the port. Very nice scenery and relaxing museum. Highly recommended. We then dined at Glady's per recommendation from someone in the castle - food was good but price very high! After that we walked back to the ship - took us about 30 minutes easy stroke with very nice view along the way. Definitely beats stuck in the traffic.