I am a big fan of learning LISP, at least once. Going through SICP after more than a decade of writing code for a living was probably the single best thing I did to deepen my understanding of a lot of compsci concepts, data structures, and how to think about software. For me, at least, it was very much a seeing the matrix for the first time kind of moment. My LISP use has quickly declined, but I've dabbled in dozens of programming languages since then, and I do attribute not feeling lost to that experience.