I'd like to go even more back to the fundamentals. Hopefully finishing SICP, building a JavaScript interpreter/compiler and other fun experiments to get a better understanding of the foundation that we're building everything on top of today.
It covers the foundation of functional languages with the simply typed lambda calculus, and also the principles of formal specifications and proofs. It's suitable for self learning, but is a significant effort so you need to have a strong interest in these topics and be ok with a very formal approach to software.