I did that cruise this past November on Grandeur of the Seas. In Aruba we went on the Atlantis Submarine which was great (but a little pricy at about $75 a person.) In Curacao we went on our own to the Aquarium, which was fantastic.
In St. Maarten we did the Beach Rendevous to Orient Beach, this included free lunch, and the food was very good.
If you're up to it in ST. Thomas we did the Kon Tiki party boat cruise with all the rum punch you could drink. Have fun!!!
I think that the best excursion in St. Maarten is the America's Cup Regatta. You go on retired America's Cup yachts and you are the crew. There are about 12-18 people on the yachts plus a very experienced crew. You can be as active or as inactive as you want. Some of the inactive roles are being the "beer babe" or a timer. It was a great time (our's won). Sign up early because not many people can go. In St. Thomas the Kon-Tiki Party Raft is fun especially if you have a bunch of people (plenty of complimentery rum punch). Also, Coral World in St Thomas is nice (an aquarium).
On St. Maarten and St. Thomas they both have a catamaran trip. One is the Champagne catamarran trip and the other is the Golden Eagle out Island sail. We really enjoyed these. We have kids and they loved it!!
We had lots of fun a few years ago on the Wild Thing speedboat trip. You get a fast boat ride, time at a beach, and party atmosphere on the way back. It used to do snorkel trips, but I think it does Magens, and goes all the way around the island.