Phoenix in the summer, for obvious reasons. Northern Arizona is fine in the summer, though, if you're not going to super crowded touristy places like the Grand Canyon. (It's the people, rather than the weather, that could make the trip unpleasant.)
New Orleans at Mardi Gras. Check the schedule online and hit some of the parades in the weeks before, enjoy some king cake, drink a Hurricane or two, then get yourself home before the drunken debauchery begins. Unless that's what you're looking for. Being pressed inside a crowd of people and having 5 types of bodily fluid from other people on me is not my idea of a good time.
Shoulder seasons are generally when I've had the best luck, although now I know that May in Canada is not good idea! Thanks!
Also, having grown up in a university town, if you're going to one, pay attention to when the students will be away for the summer, quarterly breaks, etc. Things are generally less crowded. And of course, avoid football weekends and move-in and graduation weekends! This probably matters most in a small town/big university situation like where I'm from.