I cut some large paper from the big rolls we have and had students get into pairs and put rocks on their papers. I brought around paint and squirted it all over- this was fun for me to just have freedom with this. I did not let the kids pick their colors, or where the paint was, they just had to work with what they got!
At first this was annoying to them, but they began to embrace it after awhile. Some would ask for more paint and I would ask, "What color?", their replies started out as specific colors, but then turned into "Whatever color!!!".
-J