If Canada fits your taste for adventure and you adopt an economical approach, it'd be best to split the country into 5 parts: the Maritimes (Atlantic Canada), Quebec, Ontario, the Prairies and the Mountains (Alberta / BC). Break them down into major cities and/ attractions and build a travel itinerary from that.
For first time travellers, Montreal and Toronto are mandatory on your agenda, especially in the summer. If there are two cities that pump out Canadian passion for love, life and laughter, it's definitely them. You could even limit yourself to those two cities and there would be such a long list of amazing activities within a 50 mile radius of each city that... the nike slogan JUST DO IT best sums it up. Starting in May right till September, there's a festival every weekend In and around Montreal. Toronto is Canada's economic capital, so where there's money, there's people and needless to say - things to do. Niagara Falls, a breath taking getaway is a hop skip and jump from Toronto. From Montreal, you could venture toward Quebec City and past that, you could go whale watching in Tadoussac. You can go trekking, camping, canoeing, hiking, climbing... bla bla bla... and meet a ton of amazing YOUNG people who would end up joining you or taking you here and there. If you forget staying in fancy accomodations and hit up a fellow RealJocker for help with accomodations, you'll be just fine. The bonus of visiting either city is your proximity to major US cities - Boston and New York are close to Montreal.