Strid Woods, as mentioned a few times earlier, are spectacular in autumn as much of the trees are beech.
Hardcastle Crags, near Hebden Bridge, is also very pretty in autumn and so is the stretch of the Pennine Way near High Force.
I've heard that Padley Gorge, near Grindleford, in the Peaks, is a delight and i'd imagine that the Brantingham circuit, as previously mentioned, is also worth doing at this time of year.
Autumn is a lovely time of the year for walking, my own personal favourite.