Hassel Island is a small island in Charlotte Amalie Harbour, connected to the main island by a National Park Service ferry. Most of the island is now protected as part of the Virgin Islands National Park, and it is remarkably undeveloped given its position just a few hundred metres from the cruise terminal. Ruins of British-era fortifications, Danish buildings, and the remarkable 19th-century Creque Marine Railway are scattered across its wooded hillsides.
The ferry from Crown Bay Marina runs a few times daily in season. Once there, you can walk to the fortifications and the marine railway at a leisurely pace in a couple of hours. The views back across the harbour to Charlotte Amalie are excellent, and the island feels completely removed from the commercial bustle of the main town. It is the kind of place that rewards travellers who have already done the standard sights and want something quieter.