I stick absolutely anything in my sketch books. They're fun and there is no one to say what you can and can't draw and paint. Amongst my many interests is ballroom dance and these are just some fun stuff I put in there.
If I see something by someone else that I like (and since I'm not entering it in the National Gallery again)) I quite often do my version of their work in my sketchbook for practise. It helps to learn from the masters. Here are a couple..