There are lots of great cakes in the gallery with fall themed ideas. I would do something with plenty of leaves. Maybe a cornucopia with plenty of fall produce like pumkins and gourds, maybe apples, with leaves all around. If you don't want to make it 3D or use fondant, you could do an FBCT or chocolate transfer. HTH
I have a fall cake that I just did in my pics. You could add some gourds and other fall things as well. The acorns and pumpkins were super easy to make and I just used the Wilton cookie cutters to make the leaves and then I dried them at different angles to get some movement to them. I used a variety of luster dusts to get the various colors.
I've also seen some cakes recently posted that have some really cute scarecrows on them with fall leaves and pumpkins.
Good luck!
Double post - 2nd time it's happened to me today when responding to posts! 
Hi, I just did a basket full of leaves (it is in my pics). The leaves I made out of molded chocolate and cut them with cookie cutters. It was pretty easy and they tasted great. I really didn't want to make all those leaves in fondant just so they can be thrown away.
i was thinking of a cornucopia too, but you wouldn't think more of thanksgiving for that? also what is FCBT? thanks for your help.
It is usually used just for Thanksgiving, but I personally think it's a good symbol of Fall too. All the other ideas here are great. FBCT = Frozen Butter Cream Transfer. There's an article on how to do it here: http://www.cakecentral.com/article12-How-To-Create-a-Frozen-Buttercream-Transfer.html.
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