Asahikawa is prolly the coldest spot in Hokkaido. But there's nothing more a set of long johns and 3 additional layer of clothes can take care of.
Akan, Hakodate & Asahikawa. Missed these 3 on my last trip. Otaru was quite charming.
Next time try Wakkanai, Rishiri islands and other Hokkaido tip cities. Rarely any foreign tourists (except local tourists) goes there during winter.
Japan is relatively safe so its ok even to walk the streets late at night. Walking on the path less trodden is quite ok even for a 1st time traveller methinks.