A travel backpack is a different object to a daypack. It opens flat like a suitcase rather than from the top, so you can pack it in layers and get to the bottom without emptying it, and it's cut to the dimensions airlines actually measure.

Ours run from cabin-sized to extended-trip, most with a padded laptop sleeve and a back panel that sits against your spine rather than a stack of contents. The anti-theft models add inward-facing zips and a cut-resistant strap, which is worth having if you're moving through crowded transit.

If you're flying carry-on only, one of these plus a compression cube set is usually the whole kit for a week. For longer trips they work as the second bag alongside a rolling case.