In a context where everything "low-level" is handed to us without a "how-to", looking under the hood to build your own thing is always a pleasure, be it a new programming language or a whole OS.

To attain this state of zen-itude, just look up something that you'd like to do where documentation is hard to come by ("how to write your own shell from scratch" is a funny one if you can want to learn a LOT about UNIX) and start your own project.

Related: とあるOS is a nice hobby OS too: https://github.com/klange/toaruos