For my "Teach the Class Something," I decided to show the class an old project that I worked on. I made a website for a project that my friend was doing. It was really interesting to work on and I got to play around with the visual design and I was really proud of it by the end.
I wanted to show the class this project because I wanted to give them an idea of just how powerful the tools that they were learning were. The project I made was done using only HTML & CSS and I especially liked it's visual design (of course, I'm a little biased in the matter).
It allowed me to demonstrate the idea of separation of concerns, and keeping the content and the styling/layout of the page separate. I also used it to kind of demonstrate that with not a lot of work, you can create something beautiful and informative using just HTML and CSS, and the amount of flexibility that is given with that.