You feel hanging around Sapporo is not enough to explore whole Hokkaido? If so, visiting Hakodate is recommended. Hakodate is a city located in the southern part of Hokkaido and it takes around 4 hours by train from downtown Sapporo. I honestly recommend using trans rather than renting a car unless you have special plans with a car. Even if you have a car, it takes almost same hours to get to Hakodate and weather condition in Hokkaido area is hard to expect, especially in winter. Besides, the cost is going to be similar as one way ticket from Sapporo to Hakodate costs about JPY 10,000.