Thanks to Wendy Baker, who posted this idea to the Guiding Mailing List!
Get a box of smarties. Do the following in front of the girls. Take out all the yellow ones. Split the yellow ones up between the girls. Tell them they can eat the yellow ones, but not the rest. When they ask why, tell them that you have to throw the rest out because they are "not as good because they aren't the same, they are different".
Use this activity to begin a discussion about differences, discrimination, etc.