I remember when I first learnt about Diffie–Hellman key exchange and RSA, and how cool I thought it was. So I'm thinking about going trough an example using color mixing as the secure one way function (something like this video https://www.youtube.com/watch?v=YEBfamv-_do).
Other topics I'm thinking of including is how the CA-system works, in contrast to a web of trust. Maybe also proof-of-work based blockchains or Samir's Secret Sharing algorithm.
Do you know of something cool I could share?