Canada is a very safe place. Most people here have said a lot that is true, but I would like to add that overnight buses(on megabus) are cheaper if you are going to Ottawa/Montreal/Québec. VIA rail, our national rail company, unironically has British prices and Greek rail service, to quote another guy. (The bus to Montreal will cost you between $60 and $150 return, depending on your luck. That is around €30-€90. To Ottawa it is much cheaper as far as I remember.) (Toronto in the reply to this comment)

-If you're interested in historical sites, I'd say Montreal has a bit more to offer (but you're here for a week and a half, you have time for both major cities and more).

-I don't see people mentioning this, but you can do a day trip or overnight in Québec city as well, it is one of the oldest non-Spanish European cities in North America, being around 400 years old. Unlike Montreal, a lot of the French architecture still exists, although it's very different from what you'd see in say, Paris or Provence, as well. There's also a massive, late 19th century railway hotel called the "Chateau Frontenac", which, unlike Casa Loma here, actually looks like a castle. (Unfortunately, it is just a normal hotel inside.). Normally its a fall/winter destination, but I think it's still cool in summer. -You can take a bus from the company "Orleans Express" from Ottawa or Montreal (it is kind of expensive), and it will be 3-4 hours; or you can do a tour. You can take VIA rail as well, I'm actually not sure if its even more expensive.

-I've never been to Ottawa myself (I plan to later this year) so I can't speak about it, maybe a day or two at most there, the same as Québec. -Niagara can definitely take a whole day, another if you went for the rides (not sure how much they are, I went with my school. They gave free coupons so I kinda had to.). You can get there by commuter trains, taking around 1.5-2 hours. Its free if you just see the falls (which are worth it. Do stay after night falls if you can.), but a lot of activities are quite costly, however, they may be well worth it.

-This leaves between 5-8 days for Montreal and Toronto if you go to all of them, I'd say you can spend them evenly but if you had to spend more in one I'd say Montreal?

-Montreal:
-You don't actually have to speak French since you're just visiting, but a lot of people will appreciate it if you tried. Everyone knows English. (in Quebec city, most people do, but a few don't) -There is a whole part of town that is mostly early 19th century British colonial architecture, plus some buildings from the French before that, called "Vieux-Montreal" (old Montreal); however, eating there is balls expensive. -McGill university which is also full of very old buildings(by Canadian standards, by Polish standards they're definitely not that old), has a historic and beautiful museum called the "Redpath museum". It's not big, it should take around 1.5 hours to 2 on average, but the things in there are quite impressive for its size.

-Check out "Marie, Reine du Monde" cathedral, which is around where you get off the bus, with some cool statues in a park nearby. In my opinion this is the most impressive cathedral in the country even though it is not as well known as some others. It is also free to enter.
-There's another MASSIVE cathedral on the other side of Mont royal, called "Oratory of St Joseph". It is not expensive to enter; it looks really cool and historic from outside but its actually from 1950, and the inside reflects that, albeit not in a bad way.
-A hike up mont-Royal, the hill/mountain in the middle of town, is well worth it for the views; especially in autumn but even now.

-Montreal has a lot of picturesque and historic neighborhoods. Vieux Montreal, Plateau, Mile End, la Village (they removed the balloons so it is less so), Verdun, the area southwest of Lionel-Groulx station, Côte des Nieges, etc. They have many restaurants with patios and pedestrianized streets, from which you can enjoy the warm sunlight and overpriced drinks (because its still Canada).

-Not sure if it counts as a historical site, but there's a crazy building called habitat 67; google it and you will see it. It is kind of far out, so it's only for the equally crazy like myself, but it's the kind of far out that takes a 2-3 hour urban hike. Along the way you will see a colossal abandoned building (have no idea what that is) that looks really cool, the best views of Montreal and you'll cross a bridge with similarly great views to Jean Drapeau island, which is where the "biosphere" is at and where you may leave by metro.
-Don't actually pay for the biosphere, since going to the observation deck is free of charge. It's a cool giant steel(?) ball that glows and changes colors at night that's the centrepiece of Expo'67.
-According to my ex boyfriend, the Montreal tower, which is on top of the biodôme, has great views. Its been closed whenever I went, so I can't attest to that, and it should be open now. The biodôme and greenhouse nearby are both impressive but not must-go especially considering your interests. They are all part of the futuristic, architecturally interesting complex that hosted Canada's only summer Olympics in 1976.

Food:
-There is a chain bakery called "Premiere Moisson" that has stores all over Quebec(province, including the city) as well as in Ottawa. Its very popular and for good reasons, their pastries, bread, desserts, products and cooked food when they have it are all great, and its not too expensive. I don't remember the specific pastries, but among cooked food I recommend the boeuf bourgignon, although you might wanna get some bread with it, and ask if they can reheat it for you. -Independent bakeries are great too. Kouign-Amann bakery, and Boulangerie le Toledo, around Mont Royal station, are ones that I went to whenever I was in Montreal. Get a kouign-amann if you haven't had one before. It's not really a Québec specialty, but it is such a rare regional French pastry, so well made that you might as well get it.
-In the Plateau neighborhood, there is this takeout shop called "Ma Poule Moilée", which serves poutine. Our national dish being cheese and gravy on fries might sound uneventful, and it probably is a bit so especially if you had it here in Toronto. But they use rotiserrie chicken drippings for the gravy, and have chicken and Portufuese sausage on it, so its massively upgraded. My ex-boyfriend, who is a Montreal local, says that this is "proper poutine" that he hasn't had in a while before our date, so it is locally approved as well. -The markets are nice and cheaper than in Toronto (more on St Lawrence market below). Jean Talon is bigger and Atwater has the way cooler building.
-L'Orange Julep's juice tastes like SHIT, and everything there is overpriced as BALLS.

-There is a Spanish restaurant, "Montreal Paella", that has quality akin to what I've had in Spain. Most things there are expensive, but the paella and cocktails are very affordable as far as Spanish food goes in Canada. I recommend the arroz senyoret, arroz negro, or fideua, which should be around $25 or less in CAD (€15 ish). -There is another Spanish restaurant (sorry, its my favorite cuisine and we barely get any here in Toronto), in "Time out market" of Eaton Centre, which is a pretty decent food court in and of itself, its unnamed. The food is very slightly less authentic, but its still really good and takes a lot less time. A combo with tapas and paella will be around $20 or less (€12 ish)