40 litre backpack for three weeks: the list that really worked

After years of trolley I would like to face three weeks between Thailand and Vietnam with only one cabin backpack. I have already eliminated half of the “could serve”, but between hot, rain, air conditioning and laundry I don’t understand how many shirts and shoes are really sensible. I will bring small camera and personal drugs. The goal is not record minimalism: I want a backpack manageable on trains and not have to buy useless objects along the path. I am interested in a comparison based on recent experiences…