This island fortress is accessed by a short ferry ride from the city harbour and is a brilliant day out. The island is big enough to spend several hours on and the historical features are most interesting. There are some places to eat but I recommend taking food and enjoying a picnic if it's sunny. Check out the old church as well in the centre of the island.
The Fortress of Suomenlinna is more about the foreign powers that were battling over Finland than about Finland itself. Finland gained control of the fort around the time of their independence in the early 20th century. Highlight is probably the dry dock, which is huge, and the video at the Suomenlinna museum.