I write Everything in my bullet journal! Bc I struggle w my ADD, I forget everything.. so from plans, who I talked to, what I did, tracking goals, diary, appointments, notes/reminders, shopping list, crafts and keeps going. I even have a separate journal for my diabetic helper dog- track his food meds, numbers, recipes.
I write about my day. It might sound silly, but I write about how I woke up, what feelings I'm sitting with. How I feel, what I need to put down so I can continue with my day. Now I write multiple times a day, if a thought comes and it needs to be processed it goes down to paper. Lists about things not to forget, or places I want to visit. The journal is all yours, once you get over the fact that no one is going to read it, or care to read it, you can just write. Let it all out!