From time-to-time, it's a good idea to give yourself a mental challenge, especially if you don't find yourself writing actual code on a regular basis.

This is a tic-tac-toe game created purely with HTML and CSS, without any JavaScript (or preprocessors, etc). It takes out-of-the-box thinking to create a working multiplayer game without writing any logical code. It is obviously not okay to create an application like this, but it's a cool mental exercise.