Fall is a great time to get outside and explore nature with your children. You can go to a nearby state park or just take a quick walk around the neighborhood. Have your children bring along a bag to collect their "treasures". When you return home and are warming up, read one of these books to kick the imagination into high gear.
Then let them create their own project using the items they found while on your walk. They could make a wreath by cutting out the center of the paper plate and gluing items around the edge, Modge Podge leaves to canvas, make a collage, or turn rocks into pumpkins or ghost with a little paint. Let them be creative.
Alternative: Turn the walk into a scavenger hunt. Make a list (include pictures for you preschoolers) of items for them to look for such as: an acorn, pinecone, different color and size leaves, twigs, rocks, pebbles....
Need some additional ideas, check out these links: