Ask HN: What iOS app should I make for free?

I have four entirely free days and I want to either make an entire version 1 of an app, or get a solid head start on it. I am struggling to think of a good idea at this point, and would appreciate hearing some ideas from the HN community.

I am a beginner/intermediate with Objective-C and iOS development, and I would like to make something, open source it, and submit to the App Store for (completely) free. This is more of a learning, growing, and keeping myself productive type of task.

If you need an app and want it done for free or have an idea for one, post a comment or email me (email is in my profile). Keep in mind that the app will be open sourced and free if it goes into the App Store after its completion.

I'll post a "Show HN" when the app is completed and available.