There are not a lot of things to do, if I recall but you can take a tour of the island or go to one of the local beaches (there are two beaches with the same name, the west one is the better of the two) or you can go to the Baths at Virgin Gorda. which is what most of our group did back in Oct 2000.
Yes, the Baths at Virgin Gorda Tour will be offerred by the cruiseline. They will actually make a stop for passengers booked on this excursion. The shore tour boat comes to the ship and you climb aboard. They will return you to the dock in Tortolla at the end of the tour.
We went to Virgin Gorda on the Carnival shore excursion when we were on the Paradise two years ago. It was a great excursion! We are going on the Norwegian Sky in January and wouldn't miss going back to Virgin Gorda. Totally different from any excursion on any other island.