Whatever you want!
This is what is so fantastic about art journaling.
I prefer the term art journaling over visual journaling because I think it takes the idea a bit further.
Visual journaling is basically journaling with a visual component. You can use photos, collages, drawings, painting. Writing is often in the mix as well.
Art journaling just extends the above definition into experimentation. I don't use my art journal like a diary. What I make isn't necessarily related to what is happening in my life. Well, on some level it probably is but not necessarily. I use my journal more as a place to play and experiment.
I have been a fulltime artist for twenty years. I think I was being rather too serious about the whole creating thing. I was getting as tight as a ... I don't know what. I tend to be perfectionist. Honestly not a good quality for an artist. It jams up your creativity.
It is only recently that I have discovered the art journal. This has helped to explode my creativity open.
It is a great place to give yourself permission to play. Whether you are an artist or an absolute beginner. It doesn't matter. It is a private place to make whatever takes your fancy. And for some reason, I love that it is in a book form.