I use this lab to introduce the idea of half-life. Students are given a dixie cup of M&Ms (the mini's work too and you get A LOT more in a bag). They shake the cup for 3 seconds, pour them and remove all "decayed" M&Ms, count how many remain and repeat until all M&Ms have decayed.
Students should get a great exponential decay graph. They then answer some questions based on the graph and the experiment.
You could also do this lab after teaching half-life.
Included are the instruction sheet, the student worksheet, an answer key, and sample data for students who were absent.