While in school, I made an electric scooter. I was lucky enough to be around others who already knew how to make stuff, so I was able to use their own CAD files for reference and even purchase parts similar to theirs. With the help of some very comprehensive guides and constant advice, I was able to get through a build that taught me a lot.
Without this project, I wouldn't be nearly as knowledgeable in manual machining as I am today. On the mill, I learned how to properly use an edge-finder, how to properly fixture large parts, and loads of other tricks. On the lathe, I learned how to turn multiples of the same part, how to use the tailstock properly, and lots more.
The scooter itself didn't work that well; there were loads of things that needed to be improved. I rode it a few times and recently took it apart. But that's not what matters - the things I learned while building this were easily worth the time and money I put into it.