I had looked at this thread before our trip last week and so thought I would add some addition 2 cents.

We spent Thursday and Friday skiing in Niseko (Mar 31 and April 1). It had snowed just a week before, but we saw a few days before arriving that the forecast was for a huge spike in temps, but lots of sun. It was gorgeous, pretty warm where the sun was shining but still a good chill in the air from the wind. But the snow was like slush from the early sun beating down on it (sunrise is like 5am this time of year, so 4 hours of sun already if you head out at 10am). If you are seeking out the famous powder this area has to offer, I think mid-March is the latest you should plan to go. We were largely scouting out for a family vacation either next year or the year after, so it was fun even though we had the same spring conditions we could have found right near our house...

Sapporo was an interesting stopover, we did it largely because my wife's grandmothers family is from the area and we were trying to arrange a meet up (never materialized). We didn't have much time, but liked the JR Nikko hotel and found lots of great food. It was bitter cold when we woke up the second day, wow, it's a different world from a weather standpoint than Tokyo!