A few years back I visited Toronto then took a flight to Quebec City. From here we took the train to Montreal & Ottawa and back to Toronto. Two weeks in total.
Toronto - CN Tower, Hockey Hall of Fame, Toronto island (we hired bikes & cycled around), Casa Loma, Eaton Centre, Dundas Sq, trip to Niagara Falls
Quebec City - Mainly wandered round the old town, Chateau Frontenac, Citadelle, boat trip
Montreal - old town, notre dame church, Olympic stadium, Plateau Mount Royal
Ottawa - Parliament building, Rideau canal, Canadian Mint, visited a fab museum that had totem poles but the name escapes me!
If I went again I would hire a car & try to see some more of the nature & maybe go through Thousand Islands. Would also go visit the little town where Schitts Creek was filmed!!