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St. John in the U.S. Virgin Islands is an adventure destination that doesn’t require a passport. If you want to explore the Caribbean Sea, head to the absolutely serene Trunk Bay — but go early in the morning because it gets really busy later in the day. Try the Underwater Trail, which is a snorkeling path of coral and fish with underwater signs. Boat tours, scuba diving and snorkeling are by far the most popular activities for tourists.