Cosmic Birds is a shmup with a hand-drawn aesthetic that’s become excessively popular on iOS. It doesn’t stray very far from convention, but it’s pretty tough and it ought to keep you hooked for a good few minutes. Oh, and it’s free of course.
You can earn money by defeating enemies, which can be spent on power-ups, and there are a variety of ships to unlock. It’s pretty simple, and it does the trick. So play that if you want!