Make up index cards that have descriptions of different types of people. It could cover race, religion, disabilities, whatever you come up with.
Each person has an index card placed on their back and they don't know what they have been labeled with.
Each person has to guess what their label is by the way others act towrds them.
Plan to spend quite a bit of processing afterwards to talk about why others acted towards you in a stereotypical way, and how they need to recognize these stereotypes and prejudices that they knew they had or just recognized with this activity.