Incomplete ideas, thoughts and jottings
I've carried a notebook with me for years. It's a place to make observations about the work, experiment with language, doodle and dream.
I keep it with me at all times to capture ideas. It's full of to do lists and practical things like that, but it's also full of observations about the world around me, specific people and things I may not have noticed. It's a way of staying present and observing the world.
I used to be very specific about which notebooks I would use- fancy Moleskines or leuchtturm1917's. But I have since decided the brand of notebook doesn't matter, but keeping it on you so you can make observations or jot down a couple of lines matters more. It's reducing friction between you and the habit. Now I use cheap ones, which help psychologically as I don't mind messing them up.