Some famous islands include Seurasaari , Lauttasaari and Korkeasaari - which is also the country 's biggest zoo - as well as the fortress island of Suomenlinna ( Sveaborg ) . Major islands in Helsinki include Seurasaari , Lauttasaari and Korkeasaari -- the latter being the site of the country 's biggest zoo . Other significant islands are the fortress island of Suomenlinna ( Sveaborg ) and the military island of Santahamina .