Have you used bleeding tissue paper before? This idea at Crayon Wishes and Popsicle Dreams was an activity at a Rainbow Art birthday party. The result makes beautiful stand-alone art but could also be used as a layer in multi-media art.
Are you intrigued by the idea of making your own bouncy balls? About.com shows you how to make them with borax, corn starch, and white glue. What a fun way to introduce chemistry to kids!
Making stained glass cookies is really fun and something you should try to do at least once. By using hard candies it makes the whole process easier and more kid friendly (they still get hot though!). Diary of a Mad Hausfrau has a nice tutorial for stained glass mosaic cookies and also shares this fabulous poinsettia version!