Any tips for travel in Northern Japan?

My husband and I will be spending just over 2 weeks in Northern Japan next May. We will be flying into Japan from Seoul and have not yet decided what city will fly into in Northern Japan. But we have tickets to fly out of Japan from Sapporo so that's set. We don't plan on renting a car but instead using public transit. This will be our fourth trip to Japan so we've navigated the systems previously just fine. We would love to get some advice:

• any suggestions on base cities? We thought we would choose three cities and stay about four nights in each, including 3-4 nights in Sapporo for our last few days. Any suggestions on what the two other cities should be? We would like to visit key attractions but don't plan on having if stressful trip where we're trying to do everything.

• any views just traveling by public transit in this area of the country?

• we would also like to stay a night or two at a Ryokan. Any suggestions for places in this region that would be not too difficult to get to by using public transit?

In advance, thanks for any suggestions and advice.