The fortress island is a fine example of 18th century fort building and presents a relaxed day out. You can catch the free municipal ferry just off the fishmarket in Helsinki and enjoy views of the archipelago before disembarking on the island. The island feels strangely rural with a small post office and general store, emphasising the virtual isolation the islanders must feel in winter.
You could join the Fins enjoying a swim in the Baltic or explore the parks, museums, cannons and fortress walls all over the linked islands. Try to be there when the ferry to Stockholm passes between the narrow gap between two of the Suomenlinna fortress walls!