The Ray Tracer Challenge[0] is super fun and language agnostic. It might be a good second project when learning a language. He also has a book about mazes that is great but ruby specific. Though you could adapt it to another language.

The thing I like about the books is the emphasis on having fun. My understanding is Jamis used these projects to overcome career burnout. They are also good for exploring the basics of a new language though.

I interviewed Jamis about this subject and burn out[1].

[0]: http://raytracerchallenge.com/

[1]: https://corecursive.com/025-burn-out-and-recreational-progra...