This resource provides a guide for students to discover the ratio between circumference and diameter. The students will measure the circumference and diameter of various circles with a piece of string, then create a ratio and convert to decimal. I ask the students to bring in a circle from home previous to this lesson so that there are a variety of sizes for the students to practice on.
I use it as an exploratory lesson before I introduce pi so that the students can discover it on their own.
As an extension you can get the students to average the class's results and see if the larger sample size provides results closer to pi. (Law of large numbers)
I also offer a reward for those students who were precise in their measurements and found 3.14 or a number closer to it.