This lavish turn-of-the-century island castle in the middle of the St Lawrence is only around 25km from Gananoque, but technically in the USA, so you'll need your passport to visit. It was built by George C Boldt, original proprietor of New York's famous Waldorf Astoria Hotel. Many Thousand Island cruise tours stop here, or you can drive 23km from Gananoque to Alexandria Bay, NY, USA, where 10-minute shuttles cross the 1km of water to the castle.
From the Thousand Islands Pkwy, Hwy 137/81 crosses the Thousand Islands Bridge (and the border) to Alexandria Bay. Canadian cruises depart from Gananoque for a five-hour tour (including the cruise and castle visit) and from Rockport for a 3½- or five-hour tour.