Fort George in St. Kitts is a pretty neat place to visit. I didn't book an excursion to go there, as I didn't want to be swarming over the fort with a bunch of other people at the same time from the ship, and didn't want to be on a deadline. So I just left the cruise port and went to a tourism/taxi stand right next door, asked for a cab and had a private driver take me to the fort. I forget how much it cost, something like $30-40 round trip for my wife and me, and he just hung out at the fort waiting for us until we were done.
As for the other stops, I've only been to St. Thomas, and that was basically a jewelry shopping excursion for my wife.