First get out some basic legos, mega-blocks or some such similar building toys and have groups of students create an invention.
Here is a true story to inspire them:
"Do you think you could come up with a million dollar idea? One kid did out of Legos! 7 year old Jack McKenna decides to help his mom out by creating a Lego organizer for all her beauty products. Mom loves it so much she turns it in to a real product that has turned in to a million dollar business.
Think of a problem in your house that could be solved with an invention. Build a model for it with Legos. Share with the rest of your class and get some feedback on your future million dollar idea!"
Once they have their invention, then they write a half page script trying to sell the invention on TV. Finally, they will perform their commercial for the other groups.
A variation on this is to have the students create an invention of unknown use and give it to another group. The group must figure out what the invention is used for and make a commercial for it.
Another variation is to have the groups create mysterious archeological artifacts and have the other groups figure out what the artifact is.
For more free classroom lesson ideas go to http://www.freedrama.net/lessons.html or http://www.freedrama.net/teach.html
For more free acting lessons, go to http://www.freedrama.net/acting.html