I have often been made fun of for almost always carrying a bag around with me. The main reasons why I never left anywhere without my bag were: it’s where I kept my camera, that I needed to take photos of my food, and since I have a constant runny nose, it is where I kept my tissue pack. These days, if I am not carrying around my giant camera, I have substituted my smaller camera with my smartphone, because it takes almost as good photos as my old tiny Canon camera did. I still get nagged at for carrying useless baggage around… but eh, I like my essentials close at hand.
Things I almost always have in my bag:
Wallet, phone, tissues, dental floss, lip balm, a pen (sometimes also a small notebook), a re-usable shopping bag (I am trying not to use plastic bags) and quite often there is a book to read as well.
No keys in the picture, because usually I keep them in my pockets if possible. And as I said, there always used to be a camera in my bag as well, but the smartphone has replaced it. It would be ridiculous to carry around both.
During summer, you can add a water bottle to the image.