A blazer is just as chic as a cardigan for coverage. Just be sure to go with something that won't wrinkly too much on the plane.
Neutral palletes are easiest to work with - black is an easy go to, as is grey. What makes Sienna's look pulled togther is the additional of her nautical striped shirt. It's basic and classic but also breaks up an otherwise monochromatic look.
Classic jeans and a sweatshirt but the proportions are just right. Anja really nails the casual silhouette... after all, she IS a super model.