What can I put in my microwave oven that won't get warm? I have a fuzzy understanding that microwaves heat food because the waves interact with polar molecules in the food, causing them to vibrate, and the vibrations are heat? Correct me if I'm wrong.
To this end, I placed an empty plate (ceramic presumably) for 2 minutes, and it was hot to the touch... so that was a fail.
Is there some common item/material that can be placed in the microwave oven that won't heat up proving this property of microwave oven heating?
 A: As a rough approximation, the energy pumped out by a microwave will bounce around until it gets absorbed by something.  If there is something in the oven cavity which readily absorbs energy, the energy won't bounce around very long before it gets absorbed.  If some objects in the cavity absorb energy, but not as readily, the signals will bounce around longer but still end up getting absorbed.  The trays that are included with many microwave ovens are often designed to absorb energy, but not too readily.  If nothing in the cavity absorbs energy, it may eventually get absorbed by parts of the oven itself, in ways that may damage it.  
If you want to operate a microwave oven without heating a particular object inside it, and without damaging the oven, you should place something in the oven which absorbs energy more readily than the other object.  That won't shield the object from all of the effects of the microwaves, but it will be heated far less than if it were the only thing in the cavity.
