For me, I’ve been rewriting my simple scripts and old school projects in Rust, which has been both enjoyable and informative.

Project Euler problems are pretty trivial (wrt coding) but might have some small useful nuggets. The code advent calendar from 2018 is pretty helpful too, and it’s much more code oriented than PE problems.

I’ve seen people suggest rewriting the GNU coreutils, and I really like the idea, but not quite enough to go sifting through all of the C headers and all to get to the finish line.

Honestly, it probably doesn’t matter much what you start coding as long as you’re coding.