I went to Sapporo for 6 days in X'mas.
Day 1 Arrive 札幌 (Sapporo Washington II Hotel)
Day 2 Day trip to 小樽 (620yen one way)
Day 3-4 札幌 sightseeing and shopping
Day 5 登別 - 溫泉街, 地獄谷
Day 6 Fly back to HK
www.sapporo-wh.com - in Japanese but you can check out the look of the rooms
Sapporo Washington II Hotel - it is across the street from Sapporo JR Station and it connects to the underground walk to the station so very convenient.
In 札幌 we just took metro (Mon-Fri day pass 800yen, Sat-Sun 500yen)
http://www.takimotokan.co.jp/english/index.html - 登別 Hot Spring Hotel
I took free shuttle offered by this hotel from Sapporo and then again took free shuttle back to the airport on the last day. However, if your flight to HK is a night flight, you can spend Day 6 in 登別 to visit 登別伊達時代村 / 登別熊牧場 / 登別海洋公園 NIXE and then take own transportation to the airport (about one hour by bus).