I beg to differ. Learning a new language[1] also means learning to know a different paradigm & culture & ecosystem. You learn to know novel ways of doing things

I beg to differ. Learning a new language[1] also means learning to know a different paradigm & culture & ecosystem. You learn to know novel ways of doing things that you can transfer to your main programming language---or not. Tinkering on a project in a different domain most likely means learning to know new libraries and maybe new tools. But those are most often just more of the same and don't help you to get a new perspective on what you're doing. Instead of just learning a new language, I'd also suggest to read related CS papers. [1] With "new language" I mean a language that puts emphasis on a paradigm you are not familiar with yet. E.g. if you're a ruby programmer, I wouldn't consider python a "new language"---and vice verse.