An ideal day or half-day trip from Helsinki is to pack a picnic and take the regular ferry to the island fortress of Suomenlinna Castle. A great deal of Helsinki's history was shaped here - the UNESCO World Heritage Listed fortress was founded by the Swedes in 1748 to protect against the Russians, but following a prolonged attack, Sveaborg was surrendered to the Russians in 1808. The Russians then used it to protect the shipping channels to St. Petersburg. In 1917, after the Russian Revolution, Finland declared independence and took back Suomenlinna, using it for the military until 1973 when it passed over to the citizens of Finland.
Suomenlinna is made up of 8 islands, with 3.7 miles (6 kilometers) of fortress walls, 290 buildings and over 100 guns and cannons. These days there are 6 museums, 9 restaurants and 850 residents. Many locals like to picnic among the fortress ruins with a few drinks - it can get pretty boozy here on summer weekends.
HKL ferries depart every 20 minutes from the passenger quay at the kauppatori. The Helsinki Card is valid for all ferries and attractions at Suomenlinna.