Vegas, NYC, or Chicago
Let Google help you decide www.google.ca/flights (click Places > North America) - It'll show you flight prices and things to do.
I preferred wandering around Golden Gate Park, Mission, even Chinatown over downtown.Originally posted by Manhattan
^That's where I went and it wasn't particularly memorable. Just a big skating rink surrounded by big name American department stores. It could've been a scene from any large American/Canadian city.