I bring a week's worth of thin clothes that layer. If I need more, I'm either washing by hand or using a service. Basic toiletries, an e-reader, phone, keys, charger bricks, a notebook/pens, daypack. It fits into an underseat backpack with room to spare.
This has scaled fine all the way up to multi-month scientific expeditions in remote locations. Sometimes I need to accommodate specialized equipment like formal clothes/shoes, camera bodies/lenses, excavation equipment, tent, water purification, sleeping pad/bag, etc, but that's solved through the magic of bringing a slightly larger bag.