I usually pick one of the ideas on my low-stakes "side quest" list that I maintain. Something that I won't mind if it never gets finished, or if I have to trash it and start over because of what I learned while building it the first time, now that I know the language better. I'm currently working on a RISC-V cycle-level simulator in Rust.
While I'm picking up Rust, I really like the "rustlings" project that lets you exercise various parts of the language locally, instead of in a browser, so I can use the tools that I like. https://github.com/rust-lang/rustlings
+1 for rustlings, which also inspired "ziglings" [1]. Both of these were great resources for quickly & easily learning the basics of their respective languages.