Yes, some European airlines require even smaller packs than what you mention. By my calculations, your sweet-spot is a pack with about 2,2002,400 cubic inches of capacity (remember, in a pinch a pack can be under-packed then squeezed down with compression straps). One excellent candidate would be Osprey's Kestrel 38 ($140; ospreypacks.com), which comes in at 2,300 cubic inches. Any Osprey pack will have a real hip belt, and in this case it's an excellent onea lightweight padded belt that's trim but offers good support. The Kestrel also has a clean top-loading design with not too many straps and things, and it's easy to really stuff things inside if need be. It comes in a tasteful tan/light green color scheme.
Gregory Z30 Daypack
Gregory's Z30 ($119; gregorypacks.com) puts you well inside a carry-on-size comfort range with 1,800 cubic inches, but it still has the good pack features you want, including an adjustable hip belt and lightweight shoulder straps. It also has an external "bucket" pocket for stuffing in a jacket or something like that. Sleek, and functional. And don't overlook Eagle Creek's Boundary Daypack ($85; eaglecreek.com), a nicely understated 2,000-cubic-inch pack that offers an OK suspension and plenty of travel-friendly features, such as special document pockets. It would be a solid choice, too.
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