Here's what I'd do. Look at all of the open-source applications and libraries that you use and pick the one you like best. Start helping out with that project. Really good libraries and applications usually are the ones with good open source communities behind them. And those are the projects that most benefit from new people helping out, they really carry that momentum.
Odds are that you'll be quite familiar with the library or application already. You will also have some ideas of where there are weaknesses or ideas for new features.
This advice works just as well with any language, not just ruby or python.