Create a Choose Your Own Adventure game. Here is a fancy web-based example, but creating yours as text-only, command-line based is fine. You can use ASCII art if you're feeling...wait for it...adventurous.

See the asciiart and artii gems.

Submit a pull request to this repository with all the necessary files, and replace this README with a description of your game and how to run it. Make sure to give the user instructions in the game about how to play, get help, etc.