I can no longer find that person who asked about needing a save right before the onsen trip, and I didn't understand why he needed it until I encounter it myself.
EDIT: actually just found the person.
ninjacurry if you still need it:
I realized the onsen trip is one-time only and there's no real way to access it again, so here's my save that is in the middle of the trip (eating before checking in the onsen)
It's save No.1, the 2nd on the list. Extract this directly to your LocalLow folder.
Warning: this will replace all your other saves because there's only one save.dat in it.