If it were my first visit I would take a tour with one of the many tour companies in St Thomas.
We are going again in April and again we hired Luen for our tour. He takes you into town to shop for a couple of hours, then sightseeing all over the island, then to whatever beach we want to go to. If you don't want to do the beach, he will take you back to the ship.
It's only 25.00 pp, so you can't beat that deal.
Enjoy St Thomas, it's a beautiful island.