I wanted to do this activity for a while, and this was a perfect sunny day...
There are different ways to make rainbows, but one that never fails is holding a CD/DVD against the sun. A perfect rainbow will form on the ground.
To learn how to filter the colors from the sunlight, I cut a star-shaped hole in a black cardboard paper and gave the kids colorful cellophane paper. They covered the hole with cellophane and were delighted to see how everything changes color when they look through the star. I also showed them how to create a colorful star image on the ground by letting the sunlight go through the star.
To finish this activity I prepared a Kool-Aid baking soda rainbow: I placed cool aid powder in all colors of the rainbow (and pink too) in small plastic cups, and covered it with baking soda. The kids received water in a squirt bottle and the fun began. There is something really exciting about the fizzing colorful reaction of Kool-Aid (acid) and baking soda (base) mixed together.