Kushiro is the 5th largest city in Hokkaido (I think). Most Japanese consider Kushiro the end of the earth, or similar to Siberia because of the cold weather. Summer temps are lucky to reach the low twenties (celcius) and winter gets down to the minus twenties, there's very little snow though. There's a few hundred thousand people living here, and I must say they are some of the nicest, kindest and most generous people I've met in Japan. Unlike most cold climates the people of Kushiro are very warm.
The best way to meet the locals is to head to an 'Izakaya' or a kind of home style pub. A lot of Izakaya's are actually the ground floor of someones house. Izakaya's usually have a special dish that if not unique to their pub, is often the tastiest thing on the menu. All you have to ask for is their 'o susume' (recommendation).
Just outside of Kushiro you can see Tancho (the red crested crane that is a symbol of Japan), and there are numerous lakes, untouched marshland and even an active volcano or two.
Nearby Lake Masshu is home to Marimo, little spheres of moss, perfectly formed from rubbing against the bed of the lake. See it to believe it.
Unfortunately most places require a car to reach and public transport can be expensive. I hear hitching is pretty easy though.