Highway 126 from Eugene or Florence to the turn for Whittaker Creek Recreation Area. Head south and then west on a series of BLM and USFS roads (paved and well-graded gravel). You'll need a good map and directions because of many intersections with other logging roads, lack of signs, and no cell service for smartphone GPS.
Mid- to late October is usually the peak of fall color in western Oregon.
Kentucky Falls Trail in the Coast Range mountains passes through fantastic old-growth forest to Upper Kentucky Falls, Lower Kentucky Falls and North Fork Falls. All three waterfalls are great photo ops, especially in autumn when bigleaf maple and vine maple light up the forest.