My first project with an art camp is always rainbow tie dye with the Jacquard dyes and soda ash. The kids think it is cool and they learn about caring for fibers, dying fiber and color combinations. Wrap the tables with newspaper and saran wrap, set up color stations with the dye in squirt bottles and let the students rotate around the tables after they have wound their shirts around clothespins and tied in rubber bands. I've done this with K and up - the K'ers just need help with the bands but can choose and do their own colors under close supervision.
The students love wearing their shirts to camp and pointing out the really good ones. Buy the rainbow tie-dye book that is published by Jacquard if you don't have it.