Back in the long, hot days of the summer, we met one time in the woods for the usual day of unstructured play and chance for adults to chat. One parent had the inspired idea of bringing along a white sheet, which she hung between two trees, and masses of poster paint. She had also provided brushes, sponges and old tights filled with rice.
The children worked on the sheet in quite a measured fashion to begin with, taking great pleasure in hurling the rice/tights at the sheet but also carefully making marks and prints with brushes and sticks.
They loosened up after a while, though, and soon began experimenting with different consistencies of paint as well as mixing colours and adding pouring to the list of paint application methods.
I think the result was quite spectacular in its layers, textures and complexity.