Painting on a sheet with spray bottles is an oldie but goodie. Liquid water color works well, and children can investigate the way color sinks into or runs down fabric, provoking questions for discussion. Pumping a spray bottle brings up scientific inquiry (how does it work?). Children will most certainly come up with other techniques and ideas. Teachers need to be flexible. If a girl asks to use a brush, then those brushes must come out! Should we say no to mixing color with mud and smearing it on the fabric? No! Everything is food for thought and fun.
Using "loose parts" is another way to include visual art outside. Use loose parts from nature, such as stones, logs,and "tree cookies". Include manufactured items such as tires. These entice children to create. It is more productive when teachers consciously prepare children with stories, pictures, and anecdotes inside the classroom. A child's imagination needs to be fed something beyond Ninja Turtles and Princesses! Feed their imaginations with stories about outdoor creations. The work of Andy Goldsworthy inspires all ages. Alexander Calder's early life demonstrates what children can do with found objects and scraps.
Finally, taking children to see outdoor art installations that adults have created legitimatizes this work. They need to know that what they are doing isn't just something adults have them do to fill time! Children need to see that what they are asked to do is meaningful in the adult world. Investigate your community's cultural resources. Not every town has a National Gallery of Art Sculpture Garden, but even Marfa, Texas has outdoor art!
I hope this post has given some teachers and parents not only ideas, but reasons for doing art outside with children. I can't think of a greater gift to give.