The laundry principle
Pack for one week regardless of trip length. Beyond seven days, doing laundry once is easier than carrying twice the clothing. Most cities have launderettes, most apartments have machines, and most hotels can arrange it.
What people over pack
Shoes, formal clothing for an occasion that will not happen, full size toiletries that are available at any pharmacy, and books. Two pairs of shoes is enough for almost any trip, and one of them should be genuinely comfortable for fifteen kilometres.

What people under pack
A layer for the evening, which is colder than the forecast suggests almost everywhere. A rain shell. Any medication they take regularly. A spare payment card. A universal adapter with multiple USB ports.
Cabin only, deliberately
Travelling with cabin baggage only removes the checked bag fee, the wait at the carousel, and the possibility of losing everything. Check the specific airline's dimensions, because they vary more than people assume, and weigh the bag at home.
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