Majo Bay. Go there. Don't do any of the cruise line's shore excursions in St. Maarten. Go to Majo Bay instead, get something to eat at Sunset Bar (Grouper Fingers) and watch the airliners jetblast beach-goers as they take off, and capture some breathtaking video as they come in to land. Check it out on youtube and then GO. GO THERE.
Take the taxi tour because it brings you to the spots where most of the excursions go anyways for a fraction of the cost. Additionally, once you're at those spots there's not much to do except look around for a bit so it works out in the end. You're not there too long and you got to see what everyone else saw and then some. The Dockyard is a pretty happening spot.
Dont limit yourself on this island. This is not an island where you want to spend the whole day doing something stupid like snorkeling or riding a jetski. Take a taxi island tour and experience the history and beauty of Tortola. This island has so much to offer that ships should stop here for a couple days so people can really take it all in. The views from the mountaintops are some of the best in the Caribbean. The residents of Tortola are some of the most genuine nicest people you can find and the entire island has such a laid back atmosphere about it. Please don't get caught up doing one silly thing while you're here. Don't just plunk it on a beach. There is so much to see and learn while you are in Tortola.
Your average cruise line "private island" experience. Take it or leave it. The Dragons Trail coaster is fun but short. It's entertaining watching the zip liners (they all seem to like to "air it out"). Free barbecue is okay. Save your beach plunking day for this stop as there's not much else to do.