Back when I did ballooning I learned the importance of non-synthetic clothing. Since then I make sure that jackets/vests are NOT nylon or other synthetic material. Especially in winter people seem to forget what their anoracs are made of. I managed to find a thick, cotton flight jacket in my size at an airshow once and immediately bought it... most others were all nylon.
I have a cotton overall that I used to wear for ballooning and in competitions but otherwise jeans and sneakers plus cotton t-shirt, shirt, or blouse.
Sandals I think are dangerous... even in cars they can snag the pedal. Otherwise shoes should just fit well. I once knew I pilot who used to fly in the Bahamas bare-footed at all times, and a banner pilot who flew in skirts and high heels. So what.... if it works for them.