Uusi Sauna is a stylish public sauna and bar in Jatkasaari, part of Helsinki's revival of urban sauna culture. There are separate men's and women's saunas, towels can be rented, and the bar gives it a more social feel than a purely traditional bathhouse.
It is a good choice if you want a local sauna experience without turning it into a major excursion. Check opening times before going, and expect a relaxed neighbourhood crowd rather than a polished spa atmosphere.