Here are some great resources for Lois Ehlert's Planting a Rainbow:
I found this Groovy Rainbow Cupcake from Flickr: breeleed
I made a yellow cake mix (white also works, too) and then divided the batter into four bowls. Using Wilton icing colors, I dyed each bowl of batter with my favorite color. Just spoon each color into a lined cupcake tin and you're all set!
Makes and Takes has a great activity: planting a rainbow
Angelfire has some bloomin' good plant activities!
Virtual Vine has a neat rainbow unit
Growing Vegetable Soup Activities:
First School's Vegetable Basket Craft
Cute video from Gardentime: Kids Seed Planting Projects