All the best that you can take from Helsinki is associated with the sea. Therefore, both excursions that I would recommend - sea.
Syomenlinna
Suomenlinna Fortress is located on the archipelago, called Wolf Schhers, on five hilly islands. Suomenlinna refers to the class of so-called star fortresses, which began to build in the XVIII century - they allowed the most efficient to work coastal artillery. In addition, the type includes the Petropavlovsk fortress in St. Petersburg and Hokkaido Horree. Suomenlinna was built more than forty years, moving from hand to hand and changing names. Initially, she was called Sveaborg - Swedish fortress. After in 1809, Finland became the province of the Russian Empire, Svetaborg began to call the Russian fortress. After the Finnish branch, it was renamed "Suomenlinna", which means "Finnish fortress".
Now in the fortress there is a valid prison, it is standing on the eternal joke the smallest German submarine of the Second World War in the world.
The fortress is five islands of the archipelago. Its parts connect the complex system of roads and narrow dark tunnels, according to which it is not necessary to pass without a flashlight, and ideally you should take a conductor with you. You can get around the fortress with a quick step within an hour, but to consider everything in detail and take pictures, it will be useful from three to four hours.
You can get on the excursion to Suomenlinna yourself. The archipelago with Helsinki bind the river tram (walks only in summer) and steam (goes yearly). Both types of transport are sent from the berths of the Caugtorine Square, which is two steps from the central street of the capital - Esplanada.
The fortress is allowed to walk on their own, you can hire a Russian-speaking guide. On the territory of the fortress there are museums, for the entrance to which you should pay extra: this is the Museum of Suomenlinna, Military Manege, the Museum of Toys and the Interiors of the Votko submarine.
The most characteristic souvenir that can be purchased on the archipelago is a small boat in a bottle.
Ferry in Tallina
From Helsinki to Tallinn regularly go ferries. Some firms organize one-day tours from one city to another, but tickets for a ferry from Helsinki can be bought independently. Transportation is carried out by Tallink.
Why, resting in Helsinki, is it necessary to try this excursion? Firstly, because the journey on the marine ferry is very nice. Secondly, it is a chance to see the two cities, divided by a large distance on land and very small - by sea.
Once a small town-Fortress Helsinki was created in opposition to a large rich Hanseatic city of Rivel, modern Tallinn, on the opposite bank of the Narrow Baltic Sea. And now the two capital shares only two or three hours of stroke on the ferry.
In the morning, the liner comes out of Helsinki and in the rays of the rising sun runs between Schächer - little stony islets with lighthouses and tiny houses. Do not forget to take a camera with you on the deck! During a two-hour swimming, you can do shopping in Duty Fri shops, breakfast in the restaurant or spend time with animators.
In Tallinn, before the evening ferry, you will have time to see the old town, but it will be an unforgettable walk. If there is time, be sure to go to the Kik-In-de-Kyuk Tower Museum to see one of the longest medieval guns and climb the cafe on the top platform of the tower on the medieval screw staircase. And in the evening - again in Helsinki!