If it was me I'd have 4 nights at least in Toronto - i assume you are flying into Toronto & if so, day one will be a bit of a right off! I lived in Toronto for a bit in my twenties & have been back quite a few times since. There are a number of low cost carriers I've used (all ok!) & I would fly into Toronto & out of Quebec.
I love Toronto but it's not an immediate 'wow!' city. I would second doing Air BnB & I'd go for an area like Cabbagetown or Rosedale rather than downtown. They're both attractive neighbourhoods with mostly Victorian houses & on the subway. The usual tourist shtick is the CN Tower, Centre Island, the Rogers Centre etc (Sconnotscone I'm guessing a day at the zoo must have been dragging it out a lot - it's not exactly huge!). Do get out to other neighbourhoods though - Queens Street West is arty & quirky. The Beaches (or Beach as it's also called) which you reach by streetcar is a nice stretch (,with a beach & boardwalk) along the Lake. If you are taking children, my DD liked the Black Creek Pioneer village (exactly what it sounds like - interesting for adults too!). If you want nature in the city, my favourite place was Etienne Brule park (Old Mills subway) & walk along the Hudson.
Renting cottages in the summer is very very popular with most Torontonians so if this is something you want to do then you'll need to rent a car & book early! The Muskoka area is popular as mentioned by the PP. I stayed once in Haliburton & that's great! Public transport isn't great in Canada - having said that, Niagara, Montreal & Quebec are all accessible by train (bookable in advance) & you can get to Niagara by bus.
Of the three cities, Montreal is probably my favourite. With apologies to any Torontonians, the people just seemed a lot better dressed & the men much better looking in my early twenties! It does feel a real blend of English & French cultures & from an art/culture point of view is a great place to visit. FYI most Canadians pronounce it the French way, Mon Re Al! In terms of your DS practising French, Quebecois is a dialect that has developed in relative isolation. I did go to Quebec with a Parisienne friend who did giggle quite a bit!
Quebec is lovely! Its the oldest established city in North America & it feels very European. Definetely worth a visit but maybe 2 days tops!