I've always loved the features in Offscreen Magazine that showed all of what people carry in their bags on a daily basis.
For me, its to always have a book.
Same with pens and a notebook.
But the book is a thing.
When traveling, I always pack too many books, opting for reading options over excess clothes.
When taking a book outside to read, I may bring 3–6 of the 10-15 I'm reading at the moment. Trying to better my practice of reading, by writing as I read but one step at a time.