I think this is a fantastic question, I hope you get a bunch of good answers. I do however recommend you choose a project that you are interested in, something

I think this is a fantastic question, I hope you get a bunch of good answers. I do however recommend you choose a project that you are interested in, something you can use yourself. You'll find your time more rewarding if you can "stick" with a project for longer than just "improving my skills". As such a good starting point might be something in a non-tech space, but which is a hobby of yours. For example a bird-watcher might seek out bird-watching-software and see if thats a good fit. I say non-tech only because those areas are the least served. Tech programs naturally get the lions share of techie contributers. (Although clearly not the lions share of good docs.)