I know this thread is a little on the old side, but I had to jump in here with some things to microwave that make really cool plasma:
- light a match and stick it in the microwave, put a jar over the match and microwave it for thirty seconds. You have to be really quick with this one because if you don't get the microwave going pretty soon after you get the jar over the match, the flame will go out and nothing will happen
- aluminum foil - just throw a ball in there and microwave it, or spread a sheet along the bottom and microwave that
- CD - stick it on the bottom and microwave
Don't blame it on me if you get cancer or something... and also it'll destroy your microwave, so get like a $10 microwave from Goodwill to play with.