Andy Goldsworthy is an artist who has been working with land based, site specific works in natural and urban areas. He has created some really lovely works using leaves in woodland settings, collecting leaves of similar colours and placing them together to highlight and accent certain features of the landscape around him. Two of my favorite images are shown below
(images from google image search)
I really love the yellow leaves placed around the base of the tree, there is something magical in this, like the ground where it meets the tree is alive and glowing. And the leaves placed so precisely in the water make for a very striking image.
If you are out in the woods with children, you could collect leaves and other items from the ground and create patterns with them, leaving them behind for others to find.