I dress for comfort all around.
If flying, I usually wear sweatpants, a light hoodie, and a t-shirt with sneakers or flip-flops. (Sometimes, I leave the bra in the carry-on if it's a longer flight or if it's a super late flight. Again, I don't really care and I'm wearing a hoodie anyway.) If it's warmer when I leave (which means my destination is also warmer), I wear Old Navy workout capris instead of sweats. My legs and feet are rarely cold in that situation. If I have any blisters or swelling in my feet, I wear flip-flops. I look like a loon, but I know I'm not alone.
If road tripping, I usually do the capris or jeans and a t-shirt, and I bring a hoodie with me. I learned not to wear shorts on a brief day road trip from Buffalo to Detroit (via Canada) and then Detroit back to Buffalo (via MI/OH/PA/NY) and my chafed legs (where they rubbed on the seat the whole trip) were not happy with me.