This required a lot of testing but everything on this lot should be accessible and usable for your sims. I also used TOOL by TwistedMexi to get the rocks and plants to sit into the hills and it looks so good!
The covered area near the entrance has tables and vending machines for snacks if your sim decides to spend the day here. There's a small zen garden next to it and a tea/meditation room at the top of the hill next to it.
Sims can walk along the river to the bridge that leads to the main part of the hot springs. There's a multi-story building with restrooms and sauna on the second floor and onsen showers on the first floor which also has doors that open onto the hot pools area. On this side of the river there are several hot springs that sims can relax in.
The lot traits add to the feel of an Onsen - the Geothermal lot trait is necessary or sims will swim in the pool and freeze to death during the winter lol. And as you can see in the images above, this lot has a different look for each season :D
I made a couple of cc for this but didn't have time to get the rain chain updated.