Hokkaido in late September

Hi, i am able to visit Hokkaido for 6 days from sep 28 to Oct 4 on a short notice. I want to see some fall foliage, onsen, and enjoy good seafood. Can you please suggest of an itinerary? Havent booked the flight to and from Tokyo yet so can be flexible. Many thanks for your help.

In average year your time of visit is a good time for autumn foliage in the Daisetsuzan National Park and a popular destination there is Sounkyo. It would be nice to fly into Asahikawa Airport and use Asahikawa as your base. Asahikawa is worth staying a couple of days for the best ramen in Hokkaido and the interesting Asahiyama Zoo. Your main objective is of course Sounkyo which can easily be accessed by direct bus from Asahikawa. Staying a few days in one of the hotels/ryokans in Sounkyo would surely fulfill your quest for koyo, onsen and good food.

If you are used to fast-pace travel and want to take in a bit more of Hokkaido, consider just staying 1 day each for Asahikawa and Sounkyo and then work yourself down south to Biei, Furano and Sapporo and finally fly out from New Chitose Airport.

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But ShinChitose has cheaper and more frequent flights then asahiakawa

Thanks so much for your response.
It is true that the New Chitose airport has more affordable flight to and from Tokyo.
My question now is what is the best way to get from CTS airport to Asahikawa area? Should I purchase a train pass? Thanks