You have to visit Suomenlinna "the Gibraltar of the North" when you're in Helsinki. But if you're visiting Helsinki in winter then you can leave it out because everything is under snow and it can be very cold on these windy islands. Ferry ride offers nice views to Helsinki (and the ferry is not free!). Fortress offers scary tunnels, big cannons and beautiful views to the sea and the city. Be there in the evening when the big cruise ships to Stockholm leave and past the islands (best place to watch this is the area near the King's gate). Remember that Suomenlinna is not just a museum area. There are about 800 inhabitants living on the islands and parts of them are still owned and used by military so respect their privacy! Suomenlinna has a grocery store, day care center, outdoor theatre, church, cafes and restaurants.