Look at the libraries you're using. For me, it was caolan/async. One evening, I sat down and thought about how he was doing it and I recreated all the most interesting features without looking at the code. In passing, I found a lot of opportunities to add more cool features, so I did. It was an enlightening experience, even if no one ever uses my piece of code.

It has also been mentioned by almost every single recruiter I've talked to since I posted it to Github.