What is your go-to project for learning a new language?

Ive been playing around with learning a lisp language for a while, and I recently decided to go for it and learn fennel (as I already am comfortable with lua) just to see if I like lisps as a class. Normally, I try to do the first 5-10 days of a previous advent of code to pick up a lang.

Setting up my advent of code environment got me wondering: what projects do other people use to learn new languages/tooling? Id love to hear how other people approach learning a new skill.