Suomenlinna is obviously popular amongst the expats. Checkout this post from a friend of mine :)
16.12.10 – There are a number of islands to visit around Helsinki but probably the most popular is Suomenlinna, the island fortress, which is accessible by ferry year round. I met my friends Kevin and Bazhena for lunch and after we finished eating, we decided to take a quick trip out to Suomenlinna to see what the island was like covered in snow and to see what it would be like to travel by boat through the ocean which is freezing over. It was just after 3PM but the sky was already growing dark. The ferry runs regularly, so the path cut through the ice on previous trips was still visible but it was hard not to notice the sound of the ships screws churning up huge blocks of ice as we made our way.