I have one that I keep the names of songs I want to learn in, there are so many I tend to forget, so when I want something new, I just look at my list. I also write down chord progressions and licks I come up that I want to be able to remember.

I'll also use it to write down questions I run across when I'm practicing so I can ask my teacher about them. I really think that whatever you find helpful is what you should put in it. It's your book! :)