Packing light sounds easy until the carry-on gets weighed

There is a big difference between packing for a neat photo and packing for a week of actual life. Shoes take up space, weather changes, and a laptop can use a surprising amount of the allowance before any clothes go in. Then the ticket rules turn out to be stricter than expected.

For people who travel with one small bag, what genuinely earns its place? Would love practical packing lists, especially for trips with both work and downtime. Also useful: the thing you kept bringing and eventually realized you never used.