Getting my feet wet (again)

I've decided that I need to exercise my coding muscles a bit more. I've been doing little tiny programs as part of my day job as a software tester, but it's been a few years since I've done any original, from-scratch application development.

Since I've got a Mac, and Cocoa is the environment I'm most familiar with, I'm going to start my new project as a Cocoa application. But what's the application going to be, you ask? Well, I have traditionally started out in any new environment by writing a video game, so why break with tradition?

Now, I just need to choose a basic idea...