systems programming and HDL are quite enlightening, though bordering EE. Really gets deep into how computers work and what they really are, so you are not just dealing with a compiler black box on a high level language. Being able to understand and write your own device driver is great
for maths, I think being able to quickly read binary and hex or at least quickly gauge the magnitude of a hex number is pretty underappreciated. Multi variate calculus and linear alg was also very useful in general in reasoning about programs especially those involving lists, vecs, matrices, and ML
compilers are great too but I feel like most resources and thinking around it is a bit too focused on parsing and the frontend, not enough on the "essence" of languages and compilers, abstraction, and more recent developments in static analysis other than simple typing