OOh, airports, I like that one. Good place to get a reasonable cross-section of middle and upper class people.
I never seem to hear much conversation on the subway, other than teenagers. Malls can be good, but again there's a big teenager and mom-with-kids bias. And I usually only get bits and pieces of conversation, which can be inspiring for plot but less so for character. I'd like a good place to listen to entire conversations to get to know characters better.
I'm almost tempted to get a job in an environment where you constantly overhear other conversations (eg, the lab was a good place for this compared to my current office job) because you get more of a longitudinal understanding of character. I currently work in government, and the characters all seem like the same person, very traditional and professional. Not great material for a book, unfortunately.