Geometry adventurers study circumference of a circle as they listen to Sir Circumference and the Dragon Pi, by Cindy Neuschwander. In groups, they use string to measure the diameter and circumference of circular objects and then consider the ratio of circumference to the diameter. Wrap up by teaching the class the formula for this computation and working through a couple of sample problems. if you do not have the book, find a video of it being read online.