The other day we visited Koshikawa Onsen, a tiny, old building along the 244 leading to the south-east out of Shari, Hokkaido. It is an unmanned onsen and you pay the 200 yen fee by putting the coins in a box. It seems the be mainly frequented by locals and travellers who have heard of the place. The building looks a bit run-down but the bath is nice and the water is hot.
I do treasure these tiny onsen as anybody can go to the big baths of Yufuin or Hakone, but in my romantic view this is much closer to the way onsen were used in the past before it was all about the souvenirs or the drinks. There are still the leftovers of a much older Japan out there for those who bother to look.