What do you mean by demonstration speech? Like "How to ------"?
There were some pretty dumb ones when I was in speech class a long time ago. Like "how to make toast" and the toaster the kid brought in started on fire, it was kinda funny. If I recall, mine was on how to launch a model rocket.
Do you have access to a computer with internet access and a projector? If I had to give a demonstration speech in one of my classes, it would be how to use the internet to find a good price for an airline ticket or something.