Ask HN: What are some interesting programming challenges to solve?

I have tonnes of free time as I am between jobs at the moment. I'm helping a friend with his app but that is mostly CRUD stuff.

I'm looking for interesting problems to solve using programming. Like a website that has challenges. Or something that broadens my horizon/requires brain power.

I know :- Python/C++/Golang Interested In : Functional Programming

ThePythonChallenge seemed interesting when I did it a few years ago.