In terms of Boston, have to say I thought the JFK museum was excellent and very interesting
I loved Niagara falls, it is awesome but ignore all the tackiness around it.You should do the Maid of the mist, the power of the falls is awsome, but unless you are small take a spare pair of shoes and socks with you as the plastic cover they give you only came to my knees
CN tower in Toronto is good, and if you are into sport, the Ice Hockey all of fame is good as well. Wasn't mad on Montreal, lots of 70s architecture
loved Quebec, weird, as it is dated in comparison with the rest of Canada/US but a good town for wandering, nice waterfalls north of it (Montmercy I think they are called)
You should also try and get to a baseball game, I know the game can seem silly but I love the people watching at any sporting event like that, Red Sox in Boston, Blue jays in Toronto