If you like to snorkel, you can catch a cab and take it to the Red Hook Ferry over to St. Johns/Trunk bay. St. Johns is a beautiful island, worth the time to get over there, just save some time for shopping in St. Thomas. There are also ship excersions over there, but this is easy to do on your own when in St. Thomas.
Last trip to St. Maartens, we did the America's Cup Regatta. One of the best excersions we've done so far, so if you like sailing and working the ship, this one is great! I think you can only book this through the cruiseline, its pricey, but worth the $.
Nick, my wife and I have done the Golden Eagle Catamaran in St. Maartens. We enjoyed it so much we have done it 3 times. You sail for about an hour or so, then anchor at an island. Snorkel for a few hours and then head back. Lots of fun and a great party while you sail. They even served us drinks and snacks in the water while snorkling.
We went on Golden in January & in St. Martaan did the America's Cup Regatta and agree with Donna - it was very fun and well worth it. We were lucky enough to have one of the cruisers with not much to do in the back of the sailboat & he videotaped quite a bit of it. We talked with him afterwards, gave him $10 & he sent us a copy of the videotape. If you're able, you WILL be put to work. We had a great time, but I think my husband didn't think he'd have to do quite so much work "grinding". This trip provided beer & soft drinks throughout.
In St. Thomas we did the New Waves snorkeling excursion to St. John. It was a fast, fun, (sometimes not so smooth) boat ride to 2 different snorkeling spots around the island. At the second spot, we could swim up to a beach if we wanted. They had shrimp cocktail afterwards & I believe something to drink, but I can't remember what.
We heard really good things about the catamaran snorkel and the private yacht snorkel trips. This was our first time in St. Thomas and we didn't want too long of an excursion, (not over 4 hours) so we chose the quick boat ride with more time spent snorkeling than in getting to the snorkeling spot.
We loved St. Martaan, but were not crazy about St. Thomas (I think we're in the minority there) and next time I think we would do the catamaran excurusion. (See? I'm already thinking about next time!)
I should have mentioned both of these trips were pre-arranged through Princess online - which turned out to be a good thing because on the snorkel trip, one of their boats had broken down & they could only take 1/2 of the people who had signed up & they took the ones who had signed up first.
Also, anyone who needs prescription goggles - you can get them ON the ship. Not through the excursion (at least ours), but the SHIP dive shop has them & you can rent them. I found this out too late (someone on our excursion had done this) and wasn't able to get them for this trip. I have since gone snorkeling with prescription goggles & cannot even believe what a difference it makes.
We also tried the Golden Eagle Catamaran in St. Maarten. I was fantastic!!! After we got to the island the water was a little rough for people to snorkle but we didn't care because we enjoyed sunning on the beach. They offered lots of different things to drink and had snacks on the way back. We truly recommend it.
In St. Thomas we did the St. John Island tour. It was interesting and I recommend it if you like riding in an open air vehicle and driving around the island. There were some people going to some type of beach tour that looked great also.