Trunk Bay, St. John — Photo courtesy of Karen ElowittSt. John may be only a 15-minute ferry ride from St. Thomas, but it's miles apart in terms of atmosphere and charm. The majority of the island is undeveloped National Park land, which means aside from Cruz Bay and Coral Bay, it's nothing more than pristine, undeveloped beaches and jungles. Spending a day there can be one of the best things you do while you're on vacation in St. Thomas.
Start your trip early to get the most out of the day. Passenger ferries leave at the top of every hour from the terminal at Red Hook (in the east end), and three times a day from the terminal in Charlotte Amalie. A car ferry also leaves Red Hook every half hour. All ferries disembark in Cruz Bay. To maximize your time in St. John, you'll want to get on an early ferry, so as to arrive no later than 10am.
Once you arrive in Cruz Bay, you can either take a safari bus (taxi), or rent a car to get around the island. (TIP: If you opt to rent a car, it's best to reserve it a day or two in advance during peak winter season).
I'd recommend heading to one of the North Shore beaches first, and spending the morning there. The surf is the calmest in the morning, and the light is incredible. Starting with Hawksnest Bay and extending all the way to Maho Bay, each one in this string of beaches offers unbelievably powdery white sand, absurdly clear water, and an abundance of watersports. For beginning snorkelers Trunk Bay is a favorite, as it features an underwater snorkeling trail around a small offshore cay. Cinnamon Bay is also a good option, because it has a little something for everyone: snorkeling, hiking on the marked nature trail, plantation ruins, windsurfing, and a beach bar.
For lunch, head over the hill, past Coral Bay, to Miss Lucy's, a favorite island eatery. Located on a gorgeous cove facing Friis Bay, the menu includes both American and West Indian favorites such as spicy callaloo stew, conch fritters, and kettle-cooked paella with hot sausage, chicken, shrimp and mussels over saffron rice. If you happen to be visiting St. John on a Sunday, don't miss the Sunday jazz brunch.
In the afternoon, choose one of three options:
1) Spend a day at one of the spectacular beaches on the south end of the island, such as Salt Pond Bay beach or Lameshur Bay beach. Both are quite the opposite from the white sand, picture-postcard beaches of the north shore, but instead they offer much more seclusion, and much better snorkeling opportunities (think sea turtles!)
2) Hike the Reef Bay Trail, which descends 2.5 miles through jungle and past ruins from the highest point on the island to the southwestern shore. Make an appointment for a guided tour with the National Park Service and you'll take a boat back to Cruz Bay instead of having to hike back uphill to the starting point. 
R.I. Patton Goldsmithing, Cruz Bay — Photo courtesy of Karen Elowitt3) Spend the afternoon browsing through the cute shops and art galleries in Cruz Bay. The Mongoose Junction shopping center offers a slew of fabulous boutiques and galleries such as Bamboula, Bajo El Sol, Bougainvillea, the Fabric Mill, and R.I. Patton Goldsmithing. Down in the town center you'll find the Pink Papaya, St. John Editions, and Now and Zen, in addition to numerous other boutiques offering distinctive jewelry, unique crafts, and colorful clothing.
The view from the patio at Asolare restaurant — Photo courtesy of AsolareFor dinner, my pick would be Asolare, the fantastic Asian fusion restaurant located on the hill at the Estate Lindholm Bed & Breakfast. Soak in the sunset and the spectacular view while feasting on pork spring rolls, prawn and coconut milk soup, sashimi tuna with plum sake vinaigrette, or chicken in yellow curry sauce.
