Likely, 3 summers ago, the CA$ was higher than the US$. At the moment it's the reverse, but is currently climbing back to the parity point.
Check:
http://www.bankofcanada.ca/rates/exchange/I'm sure you can understand that when US residents get a $ advantage in Canada, they are more likely to make the trip.
Re. your route, crossing into Canada at Sault Ste. Marie takes you to some very nice parts of Ontario, especially, if you stay north. & there is a recent discussion re. using Rte. 30 to go south of Montreal. Of course, I've lived in Montreal too as well as Northern Ontario, so I would like all of these places. for sure.
Oddly enough I have never made a stop in historic Quebec City, (bucket list).
I have lived down this way since 1978. All 4 provinces of Atlantic Canada have their special beauty. $ wise it costs less to take the ferry to PEI on your way working east. The bridge costs less than the ferry.
Bay of Fundy with its very high tides, Gros Morne National Park with its very special geological formations, and L'Anse aux Meadows Viking Heritage site are the "One Off" sites. There is nothing like them elsewhere on planet Earth.
:B