Day 6-8 -- just throwing out an alternative - do Sapporo --> Noboribetsu --> Hakodate, then on Day 8 return the car at Hakodate and take the train directly to CTS. It is more expensive, but instead of a boring 5-hour (yes, in reality it will be almost 5 hours than google maps' estimated 4) drive you get to relax and stretch out in the train with some pretty scenic views as it goes through onuma and winds around the east coast of uchiura bay.

In Noboribetsu I'd say book yourself into takimotokan - it's big & touristy and the in-room kaiseki dinner is "meh" at best, but the onsen is huge, has like seven different types of water, and the men's bath overlooks jigokudani.

Anyway, I did a similar trip in Fall/Autumn last year -- Itinerary on blog if you're interested to see some pics.