I wholeheartedly agree with any gilet type of jacket, as you'll probably spend a lot of time indoors standing about in queues, so it will help you stay cooler than a long sleeve jacket and you're less likely to end up having to carry it because you're too hot. Lots of pockets to keep stuff in (usually zipped), and unlike trousers, no chance that things will fall out every time you sit down.
The benefit of a jacket over cargo pants, is that you can remove it in flight, and on many carriers there is invariably a small coat hook on the seat back of the row in front, so no need to keep it out of reach in the locker. If you're a tall and well-built chap like myself, then there is nothing more uncomfortable than being jammed into your hideous charter seat with bulging pockets making your seat even more snug and uncomfortable for the next few hours.