One of the earliest places to have red leaves in Japan is Taisetsu Kogen Onsen 大雪高原温泉 at Daisetsuzan National Park (大雪山国立公園) in Hokkaido near Sōunkyō. We were there last year during late September. The Autumn colors were really quite spectacular.
During peak season you will not be able to drive to Taisetsu Kogen Onsen directly. You can park you car at a large cark park nearby and take a shuttle bus there, which departs regularly during day time. It is also quite a bit coller up there, so make sure you take enough warm clothing with you.
All photos were taken with Canon EOS 5D Mark II with EF 16-35mm f/2.8L II USM and EF 70-300mm f/4-5.6 IS USM.