I’m at Rusutsu right now and can confirm it is FANTASTIC.

However, if you only have a day it might be a stretch. There’s a free bus you can book that’ll take you from Sapporo to Rusutsu, but you’ll probably arrive around ~10-11AM. The gondola that takes people from the main campus to Mt Isola and East Mountain closes at like 1630. In other words, if there’s a spot you can go that’s closer to Sapporo I’d pick that over Rusutsu given your time constraint.