I can't talk for other cities, but Montreal is a pretty nice city to visit. There's the Old Port, the nightlife, the Botanical Garden, the people, etc. Montreal is pretty bilingual. Especially in touristic locations so you should be fine. Quebec city is also nice to visit, but its extremely French. Maybe hizzle could make some recommendations for you up there.
Toronto is pretty Americanized. I don't like it. I find it rather boring, but I guess I could see why some tourists wanna visit it. There's the Niagara falls next to Toronto. I heard great things about Vancouver. Their weather is warmer than in Montreal, but you'll catch more rain. And I heard it was more expensive too as Flipmo said.
How long are you planning on staying? The ideal thing to do is visit Canada East to West (or the other way around). But if you have limited time, you might wanna stick to Eastern Canada. I heard there was not much to do in the Prairies (my friend just moved to Calgary). But who knows, maybe British Columbia has more to offer than Vancouver.