When we think of technology, there is a tendency to limit ourselves to the high end of the tech. But, there is a great deal of science that can be best learned from low-tech applications.
Crystal radio is one of those low-tech apps. Despite advancements in radio - transistors, solid state electronics, and satellite broadcasts, to name a few, building a crystal radio provides a way for students to understand the fundamentals of communications and wave physics.
This link will take you to a page that makes an argument for including Crystal Radios into science classes, as well as including links to plans, kits, lessons, and community resources.