I like them all! However, one that I have put considerable effort into is a chemical demonstration themed on the 1812 Overture. It has 12 cannons using methanol as fuel. The original score by Tchaikovsky has the normal instruments, and then at the bottom, it actually has a line and notes for "Cannons." You can see the apparatus here:
http://www.youtube.com/watch?v=5jdxLQSEq80&feature=g-upl
It was programmed in LabView, built in our instrument shop, and spews fire like a dragon!
