Fifteen minutes by boat from Helsinki Harbour gets you to Suomenlinna. The Helsinkians seem to favour it as a leisure park: to picnic, lie in the sun and drink a lot of alcohol.
For the curious visitor, there's a lot to see though. The Suomenlinna Museum shows a good documentary about the history of the island. It's also a lovely island by itself: the fact that about 900 people still live here permanently gives it an extra edge and takes it beyond an open-air museum.