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 the subtle differences, like the difference between a fig newton or an apple newton, a ginger snap and a molasses cookie, etc. Your choice should, however, allow for more distinction than "An Almond Joy has an almond and a Mounds doesn't."
Have students first describe each treat in separate paragraphs, and then have them write a third paragraph that compares the two. Select volunteers to 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.