In the den / home office space, I changed the writing on my chalkboard to this simple fall greeting (You can find my hand lettering tutorial for that HERE), and I created this simple Fall vignette in a basket on my desk. (belleamourdesigns.com)
But I agree with you that dark mantels are beautiful with their rich warm texture and they are perfect for Fall vignettes! (atthepicketfence.com)