There are quite a number of them, depending on how long you have and what route you're taking. Covering an awful lot of ground in the process. Montreal to Cape Breton takes you through three provinces.
One recommendation (if you have the time) is from Montreal, rather than taking the big four-lane highway, there's a small two-lane highway that follows along the south shore of the St. Lawrence and around the Gaspé peninsula. Percé is at the easternmost tip of the Gaspé, but there are a number of sights from small, picturesque towns to the St. Lawrence to the ocean to the mountains to...
You can cross from Quebec into Campbellton, New Brunswick, and there again there's a small highway that follows the shoreline around the hub of NB and down into Nova Scotia. New Brunswick is well known for its covered bridges, but it also has some wonderful geography.
Nova Scotia is no slouch when it comes to beauty, either, and Cape Breton is Canada's version of Scotland. How many months did you say you had?
The last time I did that trip was on my honeymoon, in October, 2002 (today is our 33rd Monthaversary). At one picnic spot along the St. Lawrence we pulled over for a stretch, walked down to the river and encountered literally thousands of snow geese with hundreds more arriving all of the time. Amazing, and more than a little loud!
I must warn you, though. The first time I did that tour, about 13 years ago, I stayed overnight in Campbellton and left there early Sunday morning. As I was driving the back highway around New Brunswick, I found some great shots and finally used up the last of my film. Got back in the car, drove a few hundred yards, and came across a small lagoon with 37 - THIRTY SEVEN - Great Blue Herons in it. I left with the memories, but no film to record it all, and finding a camera store in rural New Brunswick on an early Sunday morning was too much of a stretch.
P.S. Drive around Cape Breton counter-clockwise. That way you'll be on the ocean side of the highway.