With autumn leaves falling outside, you should have leaves on your desktop. One of the coolest programs around is
AutumnLeaves
which causes leaves to tumble down and collected on the tops of your windows.
The occasional falling nut and growing mushroom adds to the
wet and cool feeling of a chilly autumn morning.
It’s shareware and worth paying for.

And, to add to the autumn effect, use one of
these images (
2048×1536
,
1600×1200
,
1024×768
)
to complete your leafy desktop.

By the way, the AutumnLeaves author also has
Snow for Windows
and
Hanami
(falling cherry blossoms)
so you can enjoy all of the seasons.