Cooking is a great activity to do with preschoolers. They can contribute to the group in a meaningful way and they usually feel a great sense of accomplishment when they can serve and eat food they've helped prepare. They can learn about math concepts like seriation and discrimination. They are more adventurous eaters when they have prepared the food themselves.
Here is a great article about the benefits of cooking with kids:

http://kidshealth.org/parent/growth/learning/cooking_preschool.html

Make a Smilebox recipe