Make an array of marshmallows on the glass plate.  Place them in the microwave for a few seconds until some of them begin to melt.  Measure the distance between the "hot" spots (antinodes), or the "cold" spots (nodes).  This measurement should be the 1/2 wavelength of the microwave ovens free space wavelength.  (This should be close to 6.1 cm for the 1/2 wavelength or 12.2 cm for the wavelength of the microwaves from our oven).  The frequency of the microwave should be written on the rear of the microwave oven.  In our case this is 2450 MHz.  With these a calculation of the speed of light should be possible.  

For the liquid crystal sheets:  Turn the glass tray in the microwave upside down and place the liquid crystal sheet on this.  Turn the microwave on for the time specified on each liquid crystal sheet.  The antinodes should be readily apparent when the sheets are taken from the microwave.  You can use the ruler to measure between the antinodes.