Open-air safari buses — essentially flatbed trucks with bench seating and a canvas roof — serve as the island's shared taxis, running between Cruz Bay and the major beaches (Trunk Bay, Cinnamon Bay, Maho Bay) along North Shore Road. They are cheap, frequent enough in high season, and part of the St. John experience.
For more flexibility, renting a jeep or 4WD is the best option. Note that driving is on the left (a legacy of the Danish period, retained after US acquisition), though most vehicles are left-hand drive — an unusual combination that takes a little getting used to. Roads are steep and winding in places; a 4WD is more than just a tourist affectation.
Within Virgin Islands National Park, a good network of hiking trails connects the beaches and interior sights. Many of the best parts of the island are only accessible on foot.