You could make some nice little monoprints from leaves and branches.
Just apply acrylic paint with a paintbrush onto the textured side of a
leaf. Allow students to mix and blend color variations which you can
discuss as observed in the leaves. Apply the leaves paint side down to
the surface of the paper or fabric, rub down firmly but gently and
replicate this process to create your composition.
You could take the children outside first to choose some leaves. You
could also certainly discuss the different shapes of leaves and the type
of trees based on the identification of these leaves. You could also
discuss 'why' the leaves change colors in the Fall.