Anything you would like. It is up to you. Many people put their experiences, dreams, knowledge, articles, research, pictures, and anything they can inside one.
I often keep helpful tips, and different techniques to keep with me. I use it as an ultimate guide. I have sections divided up, and I include "Lucid Dreaming" as a section, where I keep a journal of my dreams and progress, along with important techniques.
I keep in mine just different info, like on auras, tarot cards, different rituals, i use mine like a handbook for Wicca, i also keep like different item recipes like talismans and charms inside of it
