For an onsen experience, I recommend the Jozankei Onsen resort within the Minami-ku area of Sapporo, and roughly a 60-80 bus ride from Sapporo Station.
I have only ever done day visits from Sapporo. However, I have done a quick search for ryokans that accept 1 person bookings and 'Jozankei Onsen Yurakusoan' appeared.
Other than staying at an onsen, though, Jozankei Onsen also has a lot to offer for a day trip from Sapporo.
There are plenty of free foot baths dotted around the area.
And two facilities within the resort, which are my personal recommendations, are;
- 'Kokoro no Sato Jozan'. The facilities here include numerous foot baths, a lounge area where you can enjoy a selection of cakes and drinks, a library and also a tearoom.
- Hoheikyo Onsen. This onsen offers a large open air bathing experience, which can be enjoyed at all times of the year, including during the winter. (I also like this onsen, as they allow tattoos. In case tattoos are a factor for you, too).
https://livejapan.com/en/in-hokkaido/in-pref-hokkaido/in-sapporo_chito...https://livejapan.com/en/in-hokkaido/in-pref-hokkaido/in-sapporo_chito...As for other day trips from Sapporo. I'd recommend Otaru, especially for the snow light path festival. And also Asahikawa (for the penguin march at the zoo, the Asahikawa Snow festival and also for the ramen).
Another interesting spot, just south of Sapporo, is 'Makomanai Takino Cemetery', for it's buddha statue and also replicas of Stonehenge and Easter Island Moai heads.
https://hokkaidoguide.com/makomanai-takino-cemetery/