we did a 7 day eastern in November. We rented a car in both ST. Thomas and St. Maarten at the pier for about $80 with insurance. This allowed us to go to the beach for most of the day and we toured the islands on our way to the beach and back to the piers. We had no interest in shopping on either of the islands, so that was not an issue. The only thing we did plan ahead of time was to research which beaches we wanted to try.
With our family consisting of 2 adults and 3 kids, the we found the cost of excursions just to much, and found that cab fare would not be much cheaper than the car rental. The car really gave us the freedom to do our own thing on our own schedule, just make sure your back to the ship in plenty of time. Rumor had it on our cruise, someone missed getting back to the boat on time in St. Thomas.
PS. What we saved from not doing excursions, I spent on pictures.