REI Flash 18 Daypack
The Flash 18 is the pack for the trip within the trip: a light, frameless daypack that stuffs into its own pocket and comes out when you leave the big bag behind for a day. It has stayed in REI's line, revised, ever since.
Why it works. Around 250 to 400g, 18 litres, a padded back panel that doubles as a sit pad on some versions, and small enough packed to live in a corner of a suitcase. That combination is exactly what travel needs and what full-size daypacks cannot do.
What it is not. Frameless means the load rides on your shoulders, so it is comfortable to about 5kg and tiring beyond it. There is no real hip belt, and there should not be.
The category has got much better since this review, which is the update worth having. Packable daypacks used to be flimsy sacks; now several makers build them with proper fabric, a shaped back and decent straps at similar weights. If you last looked at these in 2010, look again.
Two uses people underrate: as the personal item on a flight, where it holds the things you want at your seat and then collapses into the overhead bag; and as a dry-bag liner's outer shell on a wet day, since almost nothing in this class is waterproof and a liner solves it for a few pounds.
The test before you buy: can you stuff it away one-handed? The ones that need two hands and patience get left in the hotel.
Part of the guide to backpacks and daypacks.