A big case of it depends for me.
Business Travel:
Dress pants, dress shoes, button down. Even if I don't have any meetings that day, I feel I represent my company and like to look good.
Lesiure Travel:
Spring/Summer/Fall: Jeans and a polo with tennis shoes. Rarely shorts and a short sleeve casual button down. Sometimes a windbreaker if I need it.
Winter: Jeans and a button down and my heavier coat.
International Travel:
Same as business travel, I like to be a good ambassador of the USA