Charles Street in Baltimore.

Also the Fells Point area of Baltimore, lots of fun!

Ok, maybe I was in Seattle at an unusual time, since it was fairly bustling then. Have not been to New York but have been to Toronto, Chicago, and Vegas. I am sure Toronto and Chicago would fit the bill, but I wouldn't choose Vegas alone unless you want to set the story in Vegas; it's just too recognizable, since afterall the most bustling part is around the casinos. I guess you include it in a documentary though. One aspect of urban life today is the great mix of peoples, so along with New York, Chicago, Toronto, LA, San Francisco, Miami, and maybe Vancouver would be good for inclusion.

San Antonio has a bustling nightlife with the Riverwalk district, people eat at the cafes and listen to mariachi music till 3 or 4 in the morning. A very fun and unique city.