Splash!, organized by Stanford's Educational Studies Program, is an amazing program that recruits Stanford students to teach a weekend's worth of classes to high school and middle school students on any topic imaginable. I've been involved in the program since spring quarter of my freshman year (and I regret not starting in the fall instead!).
My flagship course has been the much-enjoyed "An Introduction to Phonetics with Elvish Writing," which covers the basics of the science of speech sounds by teaching students the phonetically-based Tengwar script from The Lord of the Rings. I've also given a few tries at teaching a class on vectors and linear algebra (although I'm still working on getting that one to a comparable level of fun), and for a couple of years I co-taught a workshop in Where Are Your Keys, a game for rapid language learning that I've been addicted to since David Edwards showed it to me at an earlier Splash event.