I wouldn't take an excursion. There's an outdoor shopping mall within walking distance of the pier. Better shopping is an inexpensive taxi ride away.
One of the most beautiful places I've seen in my life is on top of the mountain in St. Thomas. You sit amid flowers and pretty greenery with a view that goes on forever. Small islands dot the ocean as far as you can see and on the other side, the cruise ships are docked below you. It's so beautiful! The day we went there we rented a car but I wouldn't really advise that. I'd just get a taxi.
When we went on our first cruise we booked a snorkle excursion to St. John and it was lovely, but from that time on, we've done things on our own and we've enjoyed it more. It's cheaper, tailored to you and there's no schedule.