If you want to try something different, you can rent mopeds/scooters from the bike shop right across the street from the pier. They were in really good condition and the driving was really easy. The locals were so polite, I never felt unsafe. You can take them anywhere. They'll give you a map and you can just go. If 2 people share a moped, it's $75 for the day. The bikes have to be returned by 5pm St. Croix time (it was 4pm ship's time). You can go to a beach and snorkel or just sightsee. We saw the rainforest, went to the Botanical Gardens, saw Sprat Hall and just basically cruised around. We could have seen a lot more if we didn't wait until 11am to get off the boat.
It was the best excursion I've ever done. I wouldn't have wanted to see the island any other way.