[edited slightly to shorten]
You can do it in the comfort of your own home! Do you have a microwave and something that melts rather easily, let’s say a chocolate bar?
The next thing you will need to know is the frequency of microwave radiation. There is a good chance it is written somewhere on your microwave oven. The standard is usually 2.45 GHz (gigahertz).
Remove the turntable first, place your chocolate bar in, and after microwaving for a bit, take your chocolate bar out and measure the distance between two melted spots.
Multiplying this distance by 2 will give you the wavelength of microwave radiation interference inside the microwave oven.
The product of the oven wavelength [what you measured] and frequency [usually 2.45, as he stated] should give you the speed of light.
I think I might try this with Alexander, who turns nine in a month.