Anything you want... journal entries, taking notes in class, doodling, composição literária stories, songs, or poems, composição literária things down that you don't want to forget, keep track of your favorito quotes, use it to make lists, make paper airplanes... the list goes on. :)
Maybe a diary, or maybe notes for school or for hobbies like if you write. Or maybe you can use it to draw, that's what my cousin does. Just doodle with it. Use it to do your homework on. Anything you want!