Yes, the trees are gorgeous in the fall...usually around mid-Oct. We have such a long, narrow state that you'll think you've been in 3 different states if you drive it from east to west or vice versa. Memphis is on the MS River, lots of flat land and farms west of the TN River. Middle TN has rolling hills and the mountains are in East TN. It takes a whole day to drive across the state in the east/west direction, too.
You mentioned SW WA.....we drove from Clarkston to Walla Walla in 1999.....then followed the Columbia River into Portland, OR. I bought local, fresh asparagus in (or near) Walla Walla and cooked it in a hot pot at the motel that night....at least I cooked what I hadn't already eaten raw.
I remember how green the area was and the contour of the hills and the little structures in the fields that we later learned were bee stations. I'll bet the Wenatchee area is pretty when the apple trees are in bloom.
You'll have to come back to TN in the fall during leaf peeping season.