On workdays, I carry a messenger bag. The zippered pockets outside are generally empty. Only the pocket on the flap holds a copy of my bus and train departure schedules both ways, just in case I need to look it up. Inside, the bag is divided in two. The back part holds my daily agenda/planner wherein I have my work schedule and other appointments, and whatever book is currently my train-book. Generally this is a thinner, easier read as I only get to read it for 15 to 20 minutes at a time so I need to be able to get in and out of it quickly. The front part holds my water bottle, my wallet, deodorant, and a little zippered bag/box thing that holds my earphones, thumb drive, locker key, a few pens, occasionally a couple tea bags and aspirin.
I also always carry my phone and keys to my house when I’m out and about. They tend to switch between being in my pockets or in my bag.
On free days, when I go places, I tend to just bring my work bag, but then without the planner, book and bag/box. And when I go light, all I bring is phone and keys with my debit card (and when necessary public transport card) in a pocket.