I think my favorite part of Bullet Journalling is Collections. These are pages for tracking special things, and lists of special things.
Things like movies to watch, or you have watched, or books to read, or have read, a savings chart for a special occasion, goals, pet vet visits, social media goals, a special visit to some where, a memory wall of a vacation, so many things can be a collection that you want to have outside of your every day bullet journalling.
They can also be placed anywhere in your bullet journal, some people just insert them on the next available page, and some set aside pages at the back of your bullet journal to keep them together. I like adding them in when the idea pops in my head about what I want to create a collection for, and then I just index it to make it easy to find. Sometimes I will even washi tape the edges of a collection to make it visually easy to get to, you could also use tabs on the side or top of your journal to mark your collections.
Because I bullet journal mostly in travelers notebooks now, I have a separate travelers notebook for my collections. Makes them easy to find, as well when I am done working in my monthly notebook, and I replace it with a new one, my collections stay in my planner.
What sort of collections do you have, or plan on having in your bullet journal?