As a middle-school CS teacher, I wrote a networking simulation [1] to help my students get a sense of how the packets get routed to their destinations. Importantly, it's vulnerable to spoofing, snooping, and censorship... students figure out how to do this mischief on their own, which leads to some great discussions of technical, policy, and ethical aspects of the Internet.

[1] https://github.com/cproctor/tcp-ip-simulation