A series of parallel lines we use for the slow race.  The idea is to have each bicyclist take a lane, everyone starts on "go" and the last person to go over the end line is the winner.  This is for demonstrating basic bike control.

We use adjacent double circles on playgrounds and connect them for a figure-8 right of way activity.  Connect the double circles with chalk to make a figure 8 with no lines in the x marking the center of the 8.  Start with one bicyclist at the top and bottom of the 8, and they ride toward the center, communicating with each other on who goes first if they reach the x at the same time.  The activity can be use for that and for teaching bike control and the "ghost rider" between each bicycle if you add more bicyclists.  

I'm assuming you already have the Oregon Bicycle Transportation Alliance's Safe Routes for Kids, a Bicycle Safety Program Curriculum, which has detailed instructions for teaching kids ~ 5th-6th grade age. It has some very good activities and lessons.