Second time for my wife and I.
Definitely plan a day to ferry to St Johns and drive around over there. The car ferry is pretty inexpensive. Food and amenities are more accessible on St Thomas and the accommodations on St Johns are more expensive. But the snorkeling is great on St Johns. Magens Bay on St Thomas is popular but a couple of beaches on the east side are pretty good too… and might be a little less crowded.
Go on a turtle dive at Buck Island. TheCatvi is who we’ve used and it is a good experience. They depart from the Marriott dock at Frenchmen’s Cove.
Take a few hours to shop in downtown Charlotte Amalie. Especially if anyone wants to look at jewelry or watches. There are a lot of types of shops though.