Inspired by the Europe thread I thought I'd ask the same about North America (yes, that includes Canada). What's worth seeing? Big cities, small cities, rural America, beaches, mountains, woods, whatever. Got any tips?

So far, potential candidates are Toronto, Vancouver, Montreal, New York, Las Vegas, Alaska, Boston, Los Angeles, Detroit, Chicago, Seattle, San Fransisco and Philadelphia. Not sure if all of them are worth seeing, but maybe you know? I would also love tips for non-city locations to explore.