Boston, Montreal and Toronto are 3 of my favourite cities.
Are you going alone, or with people? That changes what you should do IMO.
Toronto - their Zoo is pretty sweet, not a bad way to spend an afternoon with or without friends. The downtown core is pretty great for food and entertainment, just walking around is great - and if you like music, check out the MuchMusic (Canadian MTV basically) headquarters, might have a live performance or something.
Montreal - their modern art museum is great. Also, tons of shopping and the downtown/shopping district is really walkable. Always a TON of great concerts - check out the Metropolis and Club Soda venues to see if anyone is playing when you're there. Also, "La Ronde" (6 flags amusement park) is really accessible by subway and is awesome. Food: great all-you can eat sushi, and on St Denis street there's a ton of bars/clubs/restaurants - a dessert crepe place, blues bar (with awesome sangria and live music), a sweet mexican place too. Go to Schwartz, their famous sandwich restaurant. Oh, and get poutine too.
Boston - When are you going? Try to catch a baseball/basketball/hockey game or pre-season game, depending on when you're going. Im mostly drinking when I go there haha, so I'd suggest Joshua Tree as a great place to go. I never find any of the food in Boston outstanding, so I'm probably going to the wrong spots.