We had a math camp at University and needed something educational for the elementary school kids. We chose the topic graph theory and decided to teach them about planar graphs. It turns out that http://www.planarity.net has this great flash game that you can play where you have to arrange the vertices such that no edges overlap. The kids sure had fun with it. It was created by John Tantalo, a CS undergrad at Case Western Reserve University.Another task we had on paper was for the kids to design an air flight pathway between airports, where the airports are fixed ‘vertices’, and the flight paths (‘edges’) can’t overlap to avoid crashes.