Seurasaari island itself is a public park and also offers a large open air museum. Lots of people visit here in the summer months. You can get to the island by boat. There are boat shuttles taking visitors from Helsinki's south coast to Seurasaari.
The other nude beach in the area is Pihlajasaari Nude Beach and there's clothing-optional swimming at the Yrjonkatu Swimming Hall in Helsinki.
