The Thousand Islands are a group of more than 1,800 islands in the St. Lawrence River, straddling the border of the U.S. and Canada. A fashionable retreat for the elite in the late 19th century, today the area is a hub for outdoor activities. It’s home to elaborate island mansions such as the German-style Boldt Castle on Heart Island, and Singer Castle on Dark Island, with its Gothic windows and secret passageways.