Matilda K: an inhabited sea fortress built on six islands; One of the most popular tourist attractions in Helsinki as well as a popular picnicking spot for the city's inhabitants. Take a ferry ride there :)
Matilda K: At the Market Square you can find vegetables to souvenirs. Sip on a coffee and watch the bustle of the square and admire the near departing passenger ships. Maybe decide on a ferry ride to Suomenlinna