I haven't kept one lately, but I used to be an avid journaler. I kept jounalling for about five years, and I found it to be a good experience. It helped me clarify my thoughts, especially things that had been confusing me, or bits of scripture that puzzled me or that I wanted to understand better, or feelings in general.
I would write down all sorts of things, like what happened to me that day, or what I did, unusual weather events, birds or plants that I saw, scripture that hit me that day, thoughts about God, people that impacted me that day, ideas for stories, finished stories, poetry, etc. Sometimes I'd glue in a ticket stub for a moive I liked or a ball game I went to, or a picture of something I liked, or whatever.
Journalling is a great experience and I higly recommend it. It's like a record of your life.