Setup for Cookie Exercise
You'll need enough of two different types of cookies (or candies) for everyone. You may want to use two cookies/candies that are similar to encourage them to recognize and describe subtle differences, like the difference between a fig newton or an apple newton, a ginger snap and a molasses cookie, etc. It should, however, allow for more distinction than "An Almond Joy has an almond and a Mounds doesn't."
Have them describe each one in a paragraph and then write a third paragraph that compares the two. Have volunteers read or collect the paragraphs and read a few randomly. Discuss what made some essays more effective than others, and what they will need to keep in mind about effective detail in general. This is good practice for evaluating and workshopping when there's no pressure.