Check out this video on how to create your own sketchbook.
Then scroll down and look at the examples below of things that people like to practice drawing or painting in their sketchbooks.
Remember, the only limits you have on your sketchbook are the ones that you impose. You can draw, paint, cut, paste and write.
And the best part is you never have to share your sketchbook or show it to anyone unless you want to, so you can put whatever you want into it.
Some people treat their sketchbooks as daily journals and record what's on their mind, inserting items from the day (like movie ticket stubs, airline boarding passes or a napkin that they drew on at a cafe on a particularly nice day). These sketchbooks might seem more like scrapbooks or diaries.