I really love fall projects! Here in Florida we rarely see a change of seasons, so you have to first teach students what FALL is (sad, I know!) and then teach them how to do the project. It's a tough life, I tell ya!
This is a great project for third or fourth graders. The materials are minimal: brown paper towels, scraps of tissue paper, glue, and 9x12 blue paper. If you were doing this for spring you'd change the color scheme a bit, but the idea is still the same!
To make the trees and branches, you just take random scraps of brown paper towel and twist them. I had the kids glue two or three twists together to make the trunk of the tree.
Use smaller pieces of paper towel to make the branches and teeny weenie pieces to make the twigs. Yes, be sure to use the word teeny weenie.