First, I want to thank all the great folks who have posted information on this board that have made my family's previous cruise(s) so nice. Perhaps I can return the favor. Regarding doing the excursion on your own, this is the best way to do it IMHO. If you like the beach, you cant miss the beaches in St. Martin and St. Johns.
We have done the Eastern 7-day twice with our DS5 (now 6-yr-old). With the assistance of the info on this board we had a great time doing the excursions ourselves.
First St. Martin. We had the best time renting a car at the dock and driving to Pinel Island then Orient Beach. Orient Beach was beautiful but I didnt like the nudity (and I dont consider myself a prude). My BIL family was with us, 2 adults and a 6yold. It is very easy to rent a car at the dock; some of the dealers will pick you up in a golf cart to make it even easier. We liked renting a car much better than being herded around in an excursion. We loved meeting the people and having the freedom to go as we pleased. Even if that meant just laying on a hammock at the beach restaurant. St. Martin is the friendly island and the only person who approached us was to give us directions. BTW, the restaurants on the French side are incredible. I think it was the highlight of our trip. The only downer is the traffic around the French capital, which can be very slow. We didnt care for St. Martin the first time we visited on an excursion but loved it after renting a car and visiting the beaches and seeing the island on our own. It was the highlight of our cruises. Talk about memories! (Perhaps it was the gale-force winds that spoiled our first visit to St. Martin).
SIL wanted to do some shopping in St. Martin but I think shopping on vacation is a waste of time. You can get much better deals on the Internet. Of course, we got a bunch of t-shirts.
We are going on our third 7-day cruise in a few weeks. We had scheduled the Western but had to reschedule to the Eastern because of a work conflict with someone in our party. I think we will go to Mullet Bay beach this time or perhaps to Anguilla if I can talk people into it. If we only hit one beach in St. Martin, we may go to Grand Case to eat.
In St. Thomas, we first went to Trunk Bay on a beach-snorkel excursion. It was so beautiful we have returned to Trunk Bay each time doing it ourselves. Besides the beauty, Trunk Bay is a US National Park and is well developed. It also offers white sand beaches and nearby coral for snorkeling. Many beaches dont have both. To see how to do it on your own, search the board for Trunk Bay on your own. You will probably have company from others on the ship while going to Trunk Bay.
If your kids like the beach then you must go to a few of the beaches in St. Martin and St. Thomas. You will never anything like them in the states. DH will have to go with you to help you with the kids. He may enjoy it more than anyone. Let us know if you have specific questions.