One angle from which it's fun is just learning operations you can do on text (and text represented as numbers). Empirically it turns out just Shannon's "diffusion and confusion" and lots of rounds gets you far in symmetric cryptography, which we can see from a lot of simple RAX primitives working fine. But it's nice for reasons outside cryptography to get a feel for the zoo of other things you can do, what offers diffusion and nonlinearity, how you can ensure a function's reversible, etc.
If you did one topic from public-key land, I think Diffie-Hellman is easier to understand than RSA without much build-up: it makes sense from the basic properties of exponents (ones modular arithmetic doesn't change), and modular arithmetic connects to fun stuff about the primes. Merkle hash signatures also make sense with just symmetric cryptography, and are post-quantum-resistant!
I hadn't really thought about how you'd even approach PQ encryption, but maybe ECC-based McEliece is cool? You can get a feel for it even without detail on the hard decoding problem, and error correcting codes are their own interesting topic.