I attended the Wildflower Festival recently. It was a very pleasant outing, but my camera battery died after just two pictures!! There is nothing like the frustration of lining up a great shot and pressing the button and...nothing. Anyhow, the festival had a segment where they displayed some art done with natural elements. The above picture shows an interesting perspective-based art piece with trees at different distances. The piece changed depending on where you stood to look at it. And don't fret, they painted the trees with some crushed rock that would wash off in the next rain :)
The second picture I managed to get was part of a display celebrating the elements. The above picture shows the assemblage for "water".
These art pieces were very like the work of Andy Goldsworthy. If you are not familiar with him see here for some of his works. Most of his work is temporary: wind, rain, and time will dismantle it, the only record of it being the photos he takes. I like this kind of art because it is at once very child-like---most children have drawn pictures in sand, made stuff with sticks, or lined up pebbles on the ground---and also very sophisticated. It is also very ancient. Stonehenge is a colossal and long-lived example.
Here is my picture, inspired by such things: