Leggings , flats or sneakers , a top that is comfortable . Even if it has to be a tanktop & hoodie . & i don't suggest you wear something stiff like skinny jeans or heels . Think of it as something that you can sleep in during the plane ride & be comfortable in but also not be in your pajamas .
wear something basic and comfortable that doesn't wrinkle easily. choose an outfit that can easily be layered with some warmer pieces.in your carry on bag have some light but warm item that you can throw on quickly. if it's hot when you go to the airport; wear flip-flops or sandals,but have ( also in your carry on ) another pair of shoes or footwear that you anticipate wearing at your destination.