I have ridden east along the south coast of the St. Lawrence, from Lévis (across the river from Quebec City) to Rivière-du-Loup, and found the route very pleasant: not too hilly, not too far between towns, nice scenery. As a touring destination, I preferred it to the Drummondville - Sherbrooke route, which I have also done.

Going in one direction and
taking the train back is a good idea.