Oh you're going to love Half Moon Cay, paradise on Earth!
Do a sail/snorkel excursion on St Thomas to St. John. Holland America offers a ship excursion called the Champagne Catarmaran Sail & Snorkel to St. John. It is out of this world! The excursion will last about 4 hours which should leave you some time to shop in Charlotte Amalie. I've been to a bunch of different snorkeling spots in the Virgin Islands and this one is one of the best you can find if the weather is right. Pristine white sand beach and lots of coral.
I would like to book this same intinerary on the Zuiderdam someday, be sure to write a review when you come back. Have a wonderful cruise!