This is an activity that has been around a while, but without a lab write up, it isn't much help. I've written an introduction to microwaves and how they work along with data collection, a percent error calculation and a conclusion.
Students will measure the speed of light to a surprising decree of precision using a layer of marshmallows in a microwave oven. The rotating table is removed and each group heats their marshmallows 15-30 seconds. The marshmallows will rise and then sink, turning light brown, along the nodal lines. Students then measure the distance between these points, determine the wavelength, and using the frequency rating on the microwave oven, determine the speed of light.
Keywords: speed of light, microwave, cell phone, radio wave, radiowave, x-ray, gamma, electromagnetic spectrum, wavelength, frequency, speed, velocity, vacuum, Einstein, chemistry, physics, astronomy, Heisenberg, Planck.