Chicago: incredible food, great ethnic neighborhoods, gorgeous downtown and skyline, great museums, great sports...my kinda town . You wouldn't run out of things to do.

Montreal is a lot of fun, and very different b/c of French colonial influence.
Vancouver is beautiful, has lots of naturey and city stuff to offer.
San Francisco is lots of fun, and a definite must have North American city experience.
Boston very compact, very pretty, and has lots of good food, museums, sights, history etc.

If you want more info on those cities (esp Montreal, Boston, Chicago, or NYC--PM me).

Once you get through that list ...post back, and we'll have some more fun.