Trousers with a collared shirt. I usually prefer to wear a blazer or jacket of some kind. If you are dressed for business, you are usually taken more seriously and get better service. I do this even if traveling for leisure. I can always take off the jacket. I have had the unfortunate circumstance of being away from home and getting 'the call' about a relative or someone close passing away, and had to travel to a funeral or memorial service from my destination without going home first. If I am dressed like this, I can go and not worry about looking like a slob. I had to buy a suit once because I wasn't prepared.
I only wear t-shirts under another shirt or when I am doing physical work or exercising. I don't wear jeans. I like to wear western boots, but not when flying, as taking them off can be a pain. I do wear shorts in public, but only shorts that look appropriate for a classy occasion. I never wear them for air travel.
Hoodies, IMHO, make you look like a hood rat, but to each his own.
When newly minted officers arrive at the US Marine Corps Basic School in Quantico VA, they are told that you are not going to wear jeans on liberty (off base non work time). The idea is that you are an educated professional, and you are to present yourself that way at ALL times. That stuck with me and the idea has served me well for decades.
I don't want to run into anyone I know, or meet anyone important that I don't know and not present myself at my best.
Greater love hath no man than this....