So, this doesn't work by itself in most setups (it won't heat the top and the bottom of the pizza at the same time), but I've seen a really cool trick used: a plate of silicon carbide in a microwave oven absorbs microwaves and turns them into heat (just like food, but it can withstand much higher temperatures). This can reach extremely high temperatures (IIRC, 800C+), hot enough to smelt tin.
Too low, and microwaves reflect off it, going back into the magnetron, causing it to overheat and die.
Too high, and microwaves pass through it, reflect off the far wall of the microwave back into the magnetron, causing it to overheat and die.
Either do this in a microwave that has overheat protection (generally the ones which don't say in massive letters on the manual "DO NOT RUN EMPTY"), or put a glass of water in with any 'experiments'.