This website, Growing a Jeweled Rose, has so many awesome activity ideas. Here are a few that stuck out (click on the picture for the links):
Magical Hatching Dinosaur Eggs!
All images are from http://www.growingajeweledrose.com/.
EDIT: 8/11/2014: I tried Rainbow Pour Painting with my kids this weekend with Tempera paint and it didn’t work that great, definitely use the acrylic as suggested.