Place a single layer of marshmallows in microwave with turntable removed
Microwave until hotspots are visible.   Marshmallows at the hotspots will expand and melt.
Measure the distance between hotspots.   Sometimes it's easier to see them after the marshmallows are removed.
The distance between hotspots (antinodes) is one-half wavelength, so	l = 2d
The speed of light is	c = f l