Arlan sauna is an old-school public sauna in Kallio, with separate men's and women's sides and a much more local feel than Helsinki's newer waterfront saunas. It is a good stop if you want to understand the neighbourhood's everyday sauna culture rather than make a polished design pilgrimage.
The facilities are simple, and the atmosphere is part of the point. Check current hours before going, bring your own towel if required, and allow time to cool down between rounds.