In a major victory in the American Revolution, the Green Mountain Boys took this fort from the British in 1775. With costumed guides, reenactments, a museum, gardens, a maze and hiking trails, it's easy to spend a full day here.
Note that the last ticket of the day is sold at 4:30pm. Admission also includes access to Mount Defiance, 3km south and rising 758ft over Lake Champlain with panoramic views all around. There's a tour of the site at 4pm.