In November, the Louisiana Section of the American Association of Physics Teachers participated in a Physics day and a great presentation was on really simple and cheap physics demos.
Wired's Science Blogs posted details and photos about the demonstrations. There are 10 demonstrations and there are videos of them being done that you could show instead of doing the demo (although live demos are more impressive).
Check them out here:
These are cheap, easy, and require no electrical outlet. These are great ways to engage students in a topic by doing the demo before the topic, or do the demo as an explanation of a topic.