Step 1: Use a cereal box side and just freehand draw/cut out a rainbow shape.
Step 2: Get some box lids, paper plates, any "container" and then go to your craft supplies and cut/grab small items from each color of the rainbow: ROY G BIV (and add Pink if you like!)
I used red, orange, yellow, blue, green purple and pink.
Ideas: ribbon, construction paper, scrapbook paper, cardstock, wrapping paper, feathers, tissue paper, foam, foam shapes, stickers, buttons, washi tape, felt, fabric, pipe cleaners, stick-on jewels.
Step 3: Time for the kids to create! Give them the cardboard shapes, the colorful materials and a glue stick or bottle of glue and tell them to create a rainbow collage! Even if they use the exact same materials - no two will look the same - and I love that!