Meh. Doing a throwaway project just for the challenge of it is a horrible waste of time. Do something useful, if you choose your project wisely you'll learn just as much about coding, while simultaneously making the world a better place and filling out your portfolio/resume.
If you just feel the urge to learn things and improve your skills then that doesn't always require the project to have some practical utility. The person who will learn the most is the one who simply does both useful projects and projects for their own sake. Obviously with only so much time it makes sense to prioritize the useful projects, but just speaking generally here.
