Juneau, Alaska has a population of roughly 30,000. During the day, according to a taxi driver I utilized, cruise ships can bring in about 10,000 people. So during the day, the number of people in Juneau can increase by 33%. That's a pretty big "daytime bulge" in the population and it makes me wonder what metro areas have larger "daytime bulges".
I guess you can experience a similar phenomenon with employment. ie. lots of people commuting into town for their day job. But I imagine most of these people live in the same metro area even if they don't live in the same city.