I have a little C# 'game' I am working on where you can evolve images. The images are expression trees and I made al ittle "lisp" language you can edit in the game to edit your evolved images. I showed this to some 4th grade classes I was asked to talk to about programming as a career, and I let them pick what functions they wanted to try, "add, sign, multiply" etc and I would type those into the lisp and we would see what kind of images (and videos, I have a video mode too) resulted.
they had fun with it.