I would like to proffer a different strategy: Pick something that you are passionate about, good at, and comfortable with. Then push your personal boundaries and learn something new while making some app/widget/whatever, even if it's been solved/done before. Why did you do it? You wanted to stay up to date, learn something, and it was fun (and all of this will be true).
Whatever you made will probably be something pretty sweet because you chose something close to your heart, not something that was simply 'unsolved' or 'needed doing'.
Employers will love that you chose to challenge yourself to build new skills as a hobby, and because you chose something that you are passionate about, you'll probably also have a good amount of enthusiasm while you talk about it, which is equally important.
Whatever you made will probably be something pretty sweet because you chose something close to your heart, not something that was simply 'unsolved' or 'needed doing'.
Employers will love that you chose to challenge yourself to build new skills as a hobby, and because you chose something that you are passionate about, you'll probably also have a good amount of enthusiasm while you talk about it, which is equally important.