It’s a big country – how long of a train trip are you willing to take?
Quebec is lovely, Old Quebec City is my favourite place on earth. I’ve been too many times to count. What time of year are you going? Quebec can be quite cold in the winter (but so pretty.)
Montreal is a great get-a-way city. It’s full of life, arts, food, etc. There’s a comedy festival every summer, it’s massive.
Toronto has a little bit of everything, lots of shows/stage productions, symphonies, orchestras, tons of night life, museums, art galleries, sporting events, food from every corner of the world, great shopping, etc. (I live here.) I love this city, but Quebec feels more like a throwback to Europe. Toronto feels like a large (multi-cultural) North American city.
Personally, I can take or leave Ottawa. It’s not my favourite place.
All of these would be easily accessible by train (VIA in Canada, it links up with Amtrack.)
If you’re willing to go further, I highly suggest looking at Nova Scotia. I love the maritimes. The people are so friendly and warm, there’s tons of history and its well celebrated, there’s great food, stories, lots to do. I’ve found that even Halifax is a slower pace than Toronto/Quebec City/Montreal. Nova Scotia feels like coming home, everytime I visit. And I wasn’t born there. I’ve heard the rest of the Maritimes are similar, I just don’t have any first hand experience. You might want to check out Porter Airlines. Flights may be only slightly more than the train and much faster (to most of these locations.) They fly into/out of NJ. I’ve flown them a bunch of times (and I’m flying them in Dec to NYC.) They’ve always been fabulous.
If you’re willing to go to the other side of the country – I’d go to the Rockies. But I’m a skier. You’d have to beg to get me off the slopes. 😛 It’s quite a flight and not in the same price range. It’s gorgeous though.
I’d also love to get to the north – The Yukon/North West Territories. I’ve never had the chance.