With pet projects it’s possible to experience all possible aspects of a real project: idea, preparing backlog, priorities, estimations, stories, splitting tasks, tests, deployment, version control, CI/CD, UI/UX, infrastructure, design, databases etc. Not that it’s necessary to be all in one place, but it can if we want to.
Pet projects are meant to be fun. You don’t need a good idea. Pick something that you are passionate about, and make something with it. You will never run out of ideas for features or improvements this way.