Go see the beaches! They are the most beautiful in the world. Rent a jeep or catch a cab in Cruz Bay and set off, map in hand, to explore the island. Hawksnest Beach is one of the most beautiful places I've ever seen, especially in the morning. The sun hits the clear water and even standing on the beach you can see fish and rays swimming in the surf.
There are no facilities except for a port-a-potty or two and a few picnic tables in the wooded area between the parking lot and the beach so don't expect much along those lines. However, this is part of what keeps the beach itself so secluded and pristine. I could have set up camp and lived there forever.
Snorkel the Underwater Trail at Trunk Bay, VINP
This is an underwater snorkeling trail with placards describing plants, fish, etc. Its not the best snorkeling on the island, but an underwater trail is really cool!!
Snorkel on any of the beaches. If you don't like crowds, walk 20 minutes or so from the National Park service to Honeymoon beach. Best snorkeling, so far, has been at Maho (East end). I have lots of other beaches to try yet.