The thing I've found the most worthwhile is experiment with different flavours of programming languages. At first it was out of interest, then it became frustra

The thing I've found the most worthwhile is experiment with different flavours of programming languages. At first it was out of interest, then it became frustrating because I couldn't use the lesser known ones at my day job, then ultimately I incorporate much of what I learned in the context of whichever language/codebase I'm in. Examples would be dabbling in metaprogramming, functional pure/not (Haskell/F#/OCaml), lisps (Racket/Clojure), capability-based (Pony/Clean/Rust), supervised-tree actors Erlang/Elixir, simple Go, VM ecosystem jvm/.net, cross-platform Dart+Flutter, etc. Oh and if you use js, definitely invest in TypeScript. I typically have a toy db+web app that I keep rebuilding in various incarnations. I also mix in different datastores (MySQL, PostgreSQL, CockroachDB) and frameworks both server-side and client.