The other day, I picked up a free e-book on fun things you can do with your microwave.
Yes, I was the child that stuck steel wool into the microwave (on purpose). CDs are fun too, though that came later. As are marshmallows and Ivory soap (fresh Ivory soap works better). My mother's gold rim china "accidentally" went in there as well (it was ugly). I exploded eggs on purpose (my poor mother).
Obviously, anything book that promises to teach you how to make an arc welder by dismantling a microwave is up my alley. (Insert disclaimers here: all of these activities are probably not a great idea for you or your microwave).
That said, the best experiment in the book was one that was surprisingly simple-- making plasma with grapes.
You cut the grapes almost in half, so the bottom is still connected. You cover it with a glass (optional) and turn the microwave on. Behold. . .Plasma.
Here's the neat part-- the grapes are about the same length as the half-wave of 2.4GHz radiation (which is the frequency of microwave radiation). The grapes resonate and water in the juice creates ionized gas, which arcs across the grapes halves.
Here is someone's video of grape plasma with added explanations.