I think you are looking for one big (but not too big) thing you can take with you, but at Championships I picked up two small things that you might want to consider because they would be things you could actually give the kids to build and take with them.
One was based on the 4-H Junk Drawer Robotics Program and was a sandwich bag of parts that are often lying around peoples houses. Put together it will build a "clipmobile."
http://www.4-h.org/uploadedFiles/Res..._challenge.pdf (page 20)
The other was given out by a team and was a bag that builds a moving toy with a toothbrush head as one of the parts. It's called a bristlebot. The team sells them as a fundraiser, but I looked on-line and there are directions to build them in several locations. I wish I could tell you which team it was, they even won an award at Championships and the award presentation mentioned the bristlebots.