Weather is cool so not too cold.
Agree with all said so far.
Toronto:
... is a big city, not a huge amount to see.
CN tower is good, although if you can book a table in the resturant there. You can spend an hour or more enjoying good food and enjoying the view, it turns around, and you don't pay the any entry fee to go up as you are eating there.
Toronto zoo is good if you are really stuck.
Black creek Pioneer Park is very good, lots of old buildings gatherd and being run by actors to make it very authentic.
Niagra: town is tacky but falls are good, if poss try and do the "maid of the mist" boat ride, makes it all more real as you get a real sense of the greatness of the falls and the amount of water going or them. Niagra on the Lake is nice, tea shops and browsing shops.
Ottawa:
about 5 hrs drive from Toronto.
Great city, I lived there for 2 years. Plenty of museums, galleries there if into that.
National Gallery, Old Jail, Parliment buildings (do the tour),
Eatout in the city or do the various streets, loads of good food.
Gateneapark is nice that time of year.
The city Haunted walk is great fun.
Montreal:
Do the Biodome, Olympic tower, not a great many "big" attractions to visit. Nice place to eat, drink out.
Quebec:
quite a drive from Toronto about 10hrs approx.
Do the city bus tour - best way to see whats there.
Visit the fort, and Place Royale, various other old buildings, museums.
There are lots of places to stop in between travelling these bigger cities. If you do drive, then use the tourist info centres to reseve B&Bs, free service and makes it very relaxed, stayed in loads of B&Bs and all generally very high quality.
If you are staying put and not driving then you might want to consider staying Ottawa, loads more to see and do there. And you can visit Toronto, Quebec from there instead.
Other suggestion is to go to Ottawa in Winterlude, usually around mid Feb. Very cold but great fun as you can skate on the canal, do ice and snow sculpting and loads of other events take place.
HTH