I need an idea for a 48hour hackathon

Hey, I'm attending a 48 hackathon. I'm looking for some cheap idea's. I can't think of anything clever. Working with 1 or 2 other people, we will be Full Stack developers,(hopefully)Ruby, Rails, Javascript, Jquery etc. Obviously we can learn while on the project, but nothing venturing too far, like learning Java, or C++. But things like learning node, or angular, or ember are fine.

Hope that is a clear explanation of the requirements.