Two items that I always travel with, especially when flying, are compression socks and compression packers. Both are entirely different things with completely different uses when it comes to travel, but both are important.
This lightweight daypack gets thrown in my suitcase on every trip. It’s perfect for a guidebook, a sweater, umbrella, water, sunscreen and snacks. I’ve even used it in a pinch as my extra bag in lieu of a purse when flying carry-on only.