Start with a bowl of mini-marshmallows, a bowl of toothpicks, and a plate. Give your child a few ideas by constructing cubes, pyramids, and other shapes. Then, pass it over to your child and watch his imagination soar.
I've done this activity with children between the ages of 6-9 and you wouldn't believe their creations. One child made a bird, one child made a tessellation (obviously a Montessori student!) and another child made a house. Of course, the best part is eating the creation!