As an expert in traveling, I created my list of must-have things - mobile charger and power bank, medicines, credit card (cash too), pillow, water, copy of documents on phone/mail, books and many more. In order to have a good trip, plan it in advance. Here are some helpful sites travelsites.com/trip-planner/
That depends where I am going. If that's just a flight to France or Italy and I'm going to visit museums then I take as few things as possible. But if that's a hike or camping then I want to be prepared, I have a bit travel backpack and survival kit like this https://cleverwander.com/p/best-3-day-emergency-survival-kit/ , that's essential for each trip of such kind.