Thank you for the Wikipedia suggestion; I hadn't really considered using it as a learning resource because I've had bad experiences trying that with other fields. (For example, I think Wikipedia is an abhorrent place to learn mathematics, though it clearly functions well as a reference for people who already know what the hell is going on.) Perhaps being already fluent in the fundamental concepts of programming and computer science will make the Wikipedia articles on data structures & algorithms more useful from a learning perspective.
Tinkering with the source code for $language is also a great idea! That meshes with my empirical experience that the best way to learn is to "play". These days, it's rare for me to write code that isn't related to some specific personal project or business goal; maybe bringing back a bit of the "play" aspect is one of the keys to improving fluency.