Transportation:
Assuming that you're planning on staying in the downtown core and aren't planning to travel out into the more residential/less touristy areas of the city I agree that you should avoid renting a car. There's the UP Express train from the airport to Union station which is the fastest method from the airport into the city, and the train is about 5 hours to Montreal, a train change and then another 2 hours to Quebec city. It would be far less trouble and money than going back to Pearson airport.
Alternatively, Air Canada and Porter airlines fly from the island airport to Montreal, and Porter flies to Quebec city from there as well. Air Canada flies regional jets and Porter flies 70 seat propeller planes, so I would avoid them if I were traveling with a lot of baggage.
Hotels:
Toronto and Montreal have lots of hotel options. When I was last in Quebec City I stayed in the Courtyard by Marriott. What you want to see in Quebec City is the old town which is inside the fortress, so proximity to that is key. Just be warned that it's going to be a lot of walking on cobblestone roads and on a hill.
Sights:
There's a lot to do in Toronto, so without knowing what you'd like to see, I'll just make a list of notable things for you to look into.
Parks: Centre Island, High Park
Shopping: Eaton Centre / The Bay Queen Street are the traditional downtown shopping destinations.
Yorkdale Mall (Yorkdale Station on the subway) is outside downtown but has replaced Eaton Centre as the "best" shopping mall in the City.
Yorkville Neighbourhood is the high end shopping area around Bloor St between Avenue Rd. and Yonge.
Pacific Mall: Pacific Mall is a Asian styled mall in Chinatown North. It's unique in the City.
St. Lawrence Market, and Kensington Market
Attractions:
The Zoo currently has baby Pandas and a baby Polar bear but is quite far outside the city.
CN Tower: The highest observation deck in the city
Ripleys Aquarium
Rogers Centre/Skydome: The stadium where the Blue Jays Baseball team plays. There are also regular stadium behind the scene tours when it's not a game day.
Royal Ontario Museum: The major Science, Art and History Museum. While you're there a Chihuly Glass installation will have just opened. Unfortunately, you're just missing the end of the Friday Night Live season.
Ontario Science Centre: A hands-on Science Museum located in a more Northern part of the city.
Aga Khan Museum: A Museum near the Science Centre focusing on Muslim Art.
Food:
Toronto is very ethnically diverse and so there's pretty much food of every ethnicity available, as well as celebrity chef restaurants. I recommend looking at blogto.com to narrow down restaurant recommendations.
That's everything useful that I could think of off the top of my head. I hope it's useful in directing your planning.