We went to Hokkaido in 1998 as part of a group tour (sponsored by the Japanese Govt - Thank you JICA! - for ASEAN countries), and then returned in 2012. And then again in 2013.
For our last trip in 2016, we skipped Hokkaido reluctantly, because we brought our kid along - her first overseas trip - and we didn't want to be overly ambitious and visit like 5 or 6 cities... Actually, we covered 4 cities (not counting the brief stopover at Kyoto where we bought an eki-ben on our way to Tokyo/Shinjuku).
The other reason (or real reason) is that Kobe is quite a distance from Hokkaido.
But here are 35 reasons to visit Hokkaido at least once in your life. Or more.
Those are 35 places and things I haven't really done in Hokkaido. So it is on my list.