iOS app ideas for a hackathon contest

I am looking for some cool iOS app ideas for a hackathon contest. Please share if you have some suggestions. Also if there are some good sources or websites where I can check then let me know.

How about grabbing some arduinos with bluetooth and controlling them by iOS device? There’s a tutorial here.

I think hackathons are much more interesting when the phone interfaces with something, and getting a little robot or a servo or something physical happening can be a lot easier than implementing a backend.