Takabocchi Highlands

Takabocchi Highlands (or Takabotchiyama 高ボッチ山) is located between the cities of Okaya and Shiojiri in Nagano prefecture. The peak is roughly 1,665 meters high, and you can see view of the Japanese Alps, Mount Fuji, Lake Suwa and the city of Suwa. In the early morning hours between September and March, you can do the cloud gazing.

Rin Shima travelled there and camped overnight in Episode 5. This isn't a campground, just a parking lot with a washroom available. Vistors are not supposed to camp overnight like Rin did, but due to the number of tourists doing so anyhow, in 2019 the 2nd parking lot near the summit and the adjacent yard were designated as spots where setting up a tent is permitted.^[1]

The onsen mentioned in the anime could be Akanejuku in Gakenoyu Hot Springs which is a spectacular spot (it's closed when Rin arrives there in the anime).