Remove the rotating plate from the microwave, and place a (stationary) plate with a bunch of marshmallows in it.

Let the microwave run in 5-second increments until you see the first marshmallows start melting (10-15 seconds total).

You'll see they only start melting in specific spots. That because the microwaves are standing waves, and so they only provide energy where the peaks and valleys are.

(That's why you usually have the rotating plate).

Find the centers of the melting areas and measure the distance between those.

This distance is *half* the wavelength (because of how the waves oscillate). So you multiply it by two.

The frequency of the waves is usually written on a sticker on the microwave oven (it's probably around 2450 MHz).

And frequency times wavelength = speed of light.

I measured 343,000 km/s which is pretty close to the correct 299,792, considering the imprecise (and delicious) measuring device!