Via the amazing hack a day, a good web resource about things you could do with a microwave oven is provided here. Some examples (safe/unsafe experiment as they say!):
Exploding Egg/Bubbling soap/Flash Boiling Experiment/Sparking CD/Putting Household globes in a microwave oven/Fluorescent tube in a Microwave/Microwaving high-pressure sodium vapor lamps/Neon Globes in a Microwave Oven /Valves in a microwave with video/Pins and Sparks /Putting Kitchen Scourer's in a microwave/A vacuum fluorescent display in a microwave with video /A foil lined chip packet in a microwave with video/Making plasma using a coil of copper wire.
Melting Xenon Flash tubes in your microwave oven with video/Putting a LED in a microwave /Putting a pre conditioning lamp in a microwave /Carbon fibers in a microwave /Expanding Marshmallows