This could be an indoor or outdoor activity... we did it in the garage with the door open, watching the rain.
First - go on a rock hunt to find some good shapes. We went looking for smooth rocks, but only finding more jagged ones, we still made them work. Second - set up in the garage with paint, glue and googly eyes, water and rags (or paper towel, less environmentally friendly). Then paint away!! It was really fun and the rock friends turned out well!
*Don't forget that kids need to help with clean up. Maybe designate specific jobs for each child? Cleaning paint brushes, making sure the caps are on tightly and paint is put away, wipe up with wet rags etc.