Rented a car; about $90 including a temporary permit, and insurance. Mt. Gay Rum tour (good prices also), white sand and some pink sand beaches. Harrison Caves was amazing; very wet with quick growth and well-maintained.
We hired a driver for the day; $130 including tip. We saw the Pitons, the rain forest, the botanical garden and waterfall/springs, and drive-in volcano. We thought that you could get in the water at the waterfall, but this is not the case. Pretty island, but also quiet poor. I lived in New Delhi, India during the 1980's, and this reminded me of some of the conditions that I saw there.
Sunset Bar at Maho Beach. Grab a cab from the cruise terminal, taxi is $8 per person, each way. Leave about an hour to go and an hour get back; there is a draw bridge that sometimes causes traffic back up. Sunset bar has loungers to rent ($10) ok prices on food (good) and beer/drinks. Beach is a nice wading beach, with little current. Its fun to float and watch the planes fly over. On the way back, have the cab drop you in Downtown on Front Street, where you can do some shopping and then walk back to the ship (15 minute walk following signs from downtown).
St. John via ferry to Trunk Bay for snorkeling is my recommendation. We've done that before. Leave about 90 minutes each way for transportation, and set your own schedule instead of paying the over-priced shore excursion. St. John is National Park, and snorkel equipment is available to rent at Trunk Bay. There is also an underwater trail that details specifics on the reef, and a lot of marine life.