This stone castle, on Dark Island in the middle of the St Lawrence River, was built in 1905 by American entrepreneur Frederick Bourne. It's full of secret passages and hidden doors and has a dungeon – all of which you'll see on a tour. Uncle Sam runs boats from Alex Bay; Schermerhorn Harbor also visits.
Romantics may wish to check the option of staying overnight in the Royal Suite (from $800); rates include dinner, breakfast and a private tour of the property.