As far as I can tell, the only United States National Park in Canada.
OK, it's not part of the US National Park system at all, but an International Park, run by a commission with both US and Canadian members.
Campobello Island lies 30 miles west of St. John, New Brunswick, at the mouth of Passamquoddy Bay, just across from West Quoddy Head, the easternmost point in the United States. During The Gilded Age, grand hotels and cottages were constructed on Campobello, promoting the island as a summer resort for the elite families of New York, Boston, and Montreal. Among the buyers of summer "Cottages" on the island were James and Sara Roosevelt, parents of Franklin Delano Roosevelt. This became the favorite place of young Franklin as he grew up, a chance to escape the family politics of Hyde Park. Hiking and especially sailing were the order of the day.
In 1921, following his losing campaign for Vice President of the previous year, FDR brought Eleanor and his children to Campobello to spend the summer. It was during this holiday that Roosevelt was stricken with polio. Roosevelt's visits were few and brief after this, but Eleanor and the children would return frequently, and FDR would always carry fond memories of his "beloved island".
The Internatonal Park was established by an international agreement on January 22, 1964, strengthening one of the strongest, most amicable international relationships in the world. During the years that followed, the Commission purchased the surrounding cottages and the entire lower half of the island, making one part a natural preserve ad the rest a preserve of the Gilded Age.
The park now occupies 2800 acres (1134 hectares), the entire southern end of the island. It can be reached via a bridge from Lubec, Maine, or the ferry from Deer Isle.
There are really two parks, one on either side of the highway running down the middle of the island: The west side, containing Roosevelt's cottage among others, and the east side, a natural area, where you can imagine FDR leading his children on paper chases.
A visit to the west side of the park begins at the Visitor's Center, featuring a gift shop (separate cash registers for US and Canadian currency!), a few exhibits of Rooseveltiana, and a little orientation film that the guides are quite insistent you watch. Then it is on to the cottage. Along the way you will come to signs in French and English indicating the "best spot" to photograph Roosevelt's mansioncottage from. If you want to take a photo just like all of the postcards and tens of thousands of other tourist photos, feel free. A tour of Roosevelt's cottage is a much better experience, and taking pictures inside is (fortunately) discouraged. Walking out the back of the cottage, you come to trails and picnic areas. Follow one of the trails down to the pier where Roosevelt's sailing expeditions began and ended, but also where he first felt his legs go numb. Looking behind you, you will see a hillside covered with an overgroen lawn, sloping up the hill to the cottage at the top. From that spot, I imagined a stricken Roosevelt being carried up that hill by his servants, not knowing if he would be alive tomorrow.
The east side is just as interesting as the west. A series of roads will carry you to Liberty Point, where you can look back at West Quoddy lighthouse in the United States. My favorite part of the entire park was the Eagle Hill Bog, a rare New England peat bog, with dozens of unique dwarf conifers and carnivirous plants. Stay on the wooden walkway as the peat moss will swallow you up, then die of indigestion. On the far side of the bog, a path leads up a hill to an observation point from which you can see the half mile or so you have just trekked.
Campobello Island was my first experience of Canada, and although there appeared to be little fare other than fish chowder at the local senior citizens' center, the park made the trip worth the while.
Roosevelt Campobello International Park
http://www.nps.gov/roca/
Roosevelt Campobello International Park Commission Home Page
http://www.fdr.net/