Suomenlinna or Sveaborg in Swedish is a UNESCO World Heritage site. It’s an inhabited sea fortress built on the few islands in the 18th century, when Finland was still part of Sweden. Suomenlinna is my absolute favorite place in Helsinki! You can play outdoors games here, explore the fortress, have a picturesque picnic or take the sun and simply enjoy the beauty around you. There are also museums, craft shops, restaurants and summer theater.