Okay, stretching it a bit on this one as far as the art goes, but I still like the idea and it could be taken to a more artistic level if desired. Take several plastic lunch baggies (we used five) and cut a piece of paper the same length (or a little more) of the bag. Fold the paper in half over the non-zippered sides of the baggie and staple it together. Add a few embellishments and a title, if desired.
Start collecting items for the baggies. My boys decided to do a bike race to gather their items. They found several items to make a counting book.
1 baggie had 1 acorn, another had 2 leaves, another had 3 maple helicopters (samaras), 4 evergreen branches, and then 5 other pieces of nature (hickory nuts, maybe?) in the last baggie. I left the zippered pouches open overnight to let any moisture out and then zipped it up. To add to our counting book, we wrote the number of items and what each item was in a Sharpie marker on the edge of the book. This builds literacy development for younger children, too.
To make this more of an art project, it could be spruced up more, decorated with markers, have natural items glued to the "binding", etc. Let your imagination take it where you want it to go!
We had fun pulling it together! While this could certainly be done on a nature walk, the boys enjoyed their bike race nature collection!