On three occasions I surprised 4 and 5 years olds with counting on one hand to 10. here's how this is done: http://www.mathteacherctk.com/blog/2010/07/counting-on-one-hand-and-on-two/
The kids knew to count fingers in the conventional way. They could not believe it is possible to go beyond that. They tried and, when this worked, they were delighted. For a talk, I would first show that there are several ways of counting to five: bending/straightening fingers, starting with a thumb or the pinky. I would stress the point that however you count the result is always the same. After that I would count to 10.
You can prepare chocolate bars and then ask how many breaks it would take to break them into squares. It's a different way of counting the squares so the result is also the same however you break the bars: http://www.cut-the-knot.org/proofs/chocolad.shtml
Another fine activity has to do with the braid theory: draw vertical lines, join them randomly with several horizontal lines, and then follow from top to bottom alternating vertical and horizontal lines, changing direction on at every intersectionhorizontal endpoint: http://www.cut-the-knot.org/Curriculum/Algebra/Shuttles.shtml Perfect for job distribution among the kids.