Good for you! As a preschool teacher I love seeing parents cook with their kids because it fosters self esteem, shows them that they can do their part in the family work (which sets the stage for helping pick up toys), teaches basic life skills, and encourages cognitive development and fine motor skills.
One of my favorites to make with kids is "pigs in a blanket". Get a package of hot dogs (you can use low fat) and a tube of ready to bake croissant dough. Help him seperate out the dough pieces and show him how to wrap the dough triangle around the hot dog and stick it on the baking pan. It doesn't have be perfect! Then bake in the oven according to croissant directions. A little green beans and applesauce or etc. and you have a meal.
Just as a general guideline, your child can help you with just about anything that doesn't bring him too close to the stove or other hot items, or a sharp knife. (Although be cautious with electric mixers as well!)
Also, baking is a wonderful activity for preschoolers because if nothing else you can measure out each ingredient and hand it to your child to dump into the mixing bowl. Give him a big spoon so he can play at hand stirring. Once the mixer is turned on, our rule for little people is: "Hands behind your back until the mixer is off." And of couse we talk about what could happen if we poked our finger in the bowl while it was going. And constant, constant supervision.