Melt the chocolate, measure the light.

Sometimes called the "chocolate speed of light experiment" (marshmallows and cheese work too). A spinning turntable smears the standing-wave pattern evenly across a whole rotation, which is exactly why ovens have one — to heat food evenly, the opposite of what this experiment wants — so the first step is always to remove it. With the turntable out, running the oven briefly on a flat tray melts the food in stripes at the wave's antinodes, spaced exactly half a wavelength (λ/2) apart. A household microwave's magnetron runs at a fixed, nameplate-printed frequency of f = 2.45 GHz, part of the unlicensed ISM radio band, so λ = c/f ≈ 12.24 cm and λ/2 ≈ 6.12 cm (about 2.4 inches) — measure that spacing with a ruler, multiply by 2f, and you have an estimate of c using nothing but a kitchen appliance and dessert. A ruler reading off by a couple of millimetres, real magnetrons drifting a percent or two from their nominal frequency, and the melted spots themselves blurring rather than forming sharp points all add real error on top of the arithmetic, which is why the "short" and "long" presets above land several percent off despite using the same simple method. Read more about the fixed 2.45 GHz band on Wikipedia's microwave oven article.