Learning multiple programming languages. Each one gives you a glimpse into the problems that some very experienced devs thought were worth solving neatly.
Switching to Linux. The whole OS wants to teach you about its innards all the time. You can actually debug problems in software you use. And you can script all kinds of things or even start contributing to basically any application. It’s just really good at teaching and motivating programming.
Explaining things to others. It’s quite easy to have just superficial knowledge in tons of topics. And sometimes, we don’t notice, even if it’s something we do every day. Try to explain to a noob what Git is and you’ll likely struggle, both with the meta description and detail questions. If you then read up on the concepts you couldn’t explain, that will give you a much more complete image of Git and ultimately help you whenever Git becomes more complex.
This is perfect advice.