week. If I pick up my car from Hakodate airport and return at Sapporo Airport, does the car rental company charge extra.Yes. The amount differs depending on the company. For example, Toyota Rentacar will charge you 7350 yen:
http://rent.toyota.co.jp/en/rental/main06-1.html
Is the GPS in English?
I still have not heard of a car rental company that can provide English GPS. But many people find Japanese system easy enough to use.
Do the hotels in Hokkaido provide free parking space for hotel guest?
In city centers usually not. Everywhere else usually yes.