From Otaru and Sapporo to Hakodate

In the winter, I'd go for Hakodate (instead of Asahikawa for example, because you can only get to Asahikawa by car or bus). On the other hand you can use the train to get to Hakodate. I think you can make a day trip out of Otaru since it's only about 30min. by train from Sapporo.

You can find ice fishing, but it is not very likely. It's weather dependent as it affects the thickness of the ice. As you can see in this calendar: http://www.kanritsuriba.com/wakasagi/callender.html Ice fishing in Hokkaido only starts early January.