About the only part of Canada I wouldn’t recommend is Cornwall, Ontario. I spent a horrible couple of weeks there in 1994.
I’m going to mention a few spots that no one else has, but there isn’t a suggestion up above that I wouldn’t second.
Gaspé is a lovely trip - we went from Québec (City) around the Saguenay Fiord, across the St. Lawrence by ferry and then around the Gaspé peninsula. We drove no more that 100 km a day, staying in B&Bs in small towns throughout. It was fantastic - the cuisine was great, the people warm and friendly, there were things like the hang gliding in Matane and the sea kayaks throughout the lower St. Lawrence. Historical spots like Restigouche, sites of scientific interest like Miguasha - it was a great trip.
No one has mentioned Newfoundland yet, and that needs to be brought up. St. John’s has a great assortment of bars and night spots, there are beautiful areas to hike, such as Gros Morne, much of historical interest such as L’Anse aux Meadows. The ocean is a massive part of their culture, though there is much natural beauty to explore away from the coast. Oh, and the people are extraordinarily generous.
No one has yet mentioned Manitoba. Winnipeg features one of the most interesting arts scenes in Canada, despite a perpetually bad economy. Then there are beautiful spots like Riding Mountain National Park and the Little Saskatchewan River. Churchill is famous for the polar bears, but throughout the north of the province there is much to be admired - chains of rivers and lakes as you approach the tree line. Lots of white water for kayaking.
Prince Edward County, in Ontario. This is where many of the United Empire Loyalists ended up in the 1780s and there is a lot involving the history of the region to be explored. This is also one of the interesting areas for cuisine in Ontario, with cheese-makers, wineries, breweries and cideries all contributing to the burgeoning local cuisine. Arts festivals, music festivals and a year-round theatre scene, and an interesting sense that the tourists that have found the place like it the way it is and don’t want in to change like some of the more touristy areas of Ontario (think Niagara, which is beautiful but becoming over-done.)
I just saw someone’s pictures from the Dempster Highway last night - he had ridden up to Fort MacPherson, NWT on a dirt bike. The Qu’appelle valley in Saskatchewan. The Algoma Canyon in Ontario. Hiking from Banff townsite to Jasper townsite in Alberta. All the fantastic places in BC. Halifax, Wolfville and the rest of Nova Scotia. Cruise ships along the Davis Strait.
That’s just a few quick recommendations, but there are so many beautiful and interesting spots in Canada, it’s difficult to narrow it down to just one region. Particularly if your tastes run to the natural and the isolated.