This would be a fantastic activity to do with school age kids. It's art and science all in one! It's affordable, too, at $4.95.
From the Innovatoys website: "Sun, water, a bit of imagination and our Sunprint Paper Kit, that’s all you’ll need to create your own work of art! Here’s the photographic process: Place an object on the Sunprint Paper and dry it in the sun no longer than half an hour. When the paper is exposed to light, it goes through a chemical change. Now soak the Sunprint Paper in water to save the print, then dry again and enjoy what appears! In fact, you can place a fern, flower, animal or any other objects on our paper and turn them into long-lasting images."
You can also buy refill paper here.