The area around Montreal and Quebec is amazing for a holiday. Both a great cities, worthy of a few days for exploration. We visited a range of other places in the area, all of which I'd recommend...
Mont Tremblant area - a really beautiful area, lots of lakes and mountains, and small towns/villages.
Lac Saint Jean - huge lake with lovely places all around it
The fjord off the Saguenay river - loads of great walking and wildlife watching (whales etc) - incredible scenery
There's also the Algonquin national park (haven't been, but apparently very nice), and Ottowa (also heard good things).
We went to Toronto on another trip. I know people rave about it, and I do remember having a nice time, but I don't remember anything specific enough to recommend about it, and I didn't come away thinking 'I must go back' or 'I must tell other people about it'.
There is so much to see and do in Canada (and N America more widely), but if you're already tied to Montreal/Quebec, I would most definitely stay in that area rather than going further afield. It was one of my favourite holidays.