While the trip was fun, interesting ("totemo tanoshikatta desu!"), it could have been improved.
Tulips and Pink Moss
Knowing what we know now, the next time we come to Hokkaido to see the flowers, we will make Mombetsu (or Monbetsu) our starting point. Both Kamiyubetsu Tulip Park and Takinoue (ta-ki-no-wear) are closer to Mombetsu than Abashiri or Asahikawa.
That said, we did love the rather quiet Abashiri.
It may be better to fly into Mombetsu, rent a car and drive from Mombetsu to Takinoue, Yubetsu, Saromako, and the lake areas to the Northwest, and come back to Engaru (if we want to go on to Asahikawa via train) or back to Mombetsu, if flying out.
Best time to go, probably mid-May onwards for the tulips and phlox.
April would be a good time to visit Kawaguchiko as they have the pink moss (phlox) at the time too during their flower festival. And Mt Fuji would have pink shoes at her feet. The Mt Kachi Kachi Ropeway will be a must on our next trip, if the weather is fine and Mt Fuji is visible.
If climbing Mt Fuji is the objective, should go in July when the trail opens.
Onuma Park - Mt Komagatake
Next time we may want to go up to Mt Komagatake near Onuma Park. If possible.
Also not necessary to rent a car, the train goes to Onuma. Just need to check if there are hotels there, or to camp there. There's Crawford Inn, but it's a western-style hotel.
There seems to be a station hotel also. Camping in Onuma Park is also an option, though the logistic may be more interesting, to say the least.
Onuma Park will be lovely in Autumn (October). Should aim for October.
This looks interesting for a day-hike. We can use Sendai as the base (Hotel there), early morning day trip to Omoshiroyama-Kogen. Hike to Yamadera (about 2 hours, 6 km). Take a train back.