Here's what my kids did last weekend!! We saw this on Instructables. It's called a Kid Wash 2. It's just a mister to walk, run, ride your bike through on hot days. My son is 14, my daughter is 11 and they spent about three hours total on this with minimal help from dad.
I think we spent about 15.00 in parts at the most. It works great and everyone loves it!!
It's a little hard to see the water coming out but you can sure see the rainbow. The link above will take you right to the instructions.