Sounds like you're already off to a good start coming up with sketchbook ideas! I usually use mine super casually, filling it with the messier studies of whatever I'm trying to learn, like simplified anatomy shapes or perspective boxes. While you do have control over whatever you put in it, the biggest key is to not be afraid of having "rough" sketches and to condition yourself to regularly work in it as often as you can without exhausting yourself. I've actually also gotten a small bag for my sketchbook to carry with me so I can draw people wherever I go like restaurants and malls. It's a really good way to run into things to practice you haven't done before, and it teaches you to be more observational and break down complicated forms. Hope this gives you some ideas!