Helsinki, Finland + Add a trip

Take a trip to Hvitträsk that was designed to be a studio home for the members of Gesellius, Lindgren, and Saarinen. It later became the private residence of Eliel Saarinen. It is located about 18 miles from Helsinki in Kirkkonummi, Finland. It was build in1902. http://www.nba.fi/en/hvittrask_en. Or take a trip to Sibelius home - Ainola in Järvenpää, 20 miles outside Helsinki. http://www.ainola.fi/eng_index.php

If weather is nice & warm, go for a picnic at Kaivopuisto park. You can also go bobsleighing there when there's enough snow on the ground. Then walking around in Eira & Punavuori district, looking at Jugend style houses and dropping by into small boutiques (if it's Saturday) in the Design District area: http://www.designdistrict.fi/design_district

If you like flea markets, weekends are a good time to go check them. Like Valtteri flea market in Vallila, for example.

Summer time the Suomenlinna fortress island is an excellent half a day trip directly from the heart of the city by ferry for 20 minutes. In winter, the Mannerheim museum or Gallen-Kallela museum. Especially the G-K bathtub location is interesting. As well as G-K café has an excellent selection.

If you like museums, Kiasma for modern art or the National History Museum are a good way to spend your time. Otherwise, I agree that walking through Aleksanterinkatu and the Esplanadi promenade year-round are nice ways to spend your time.

In summer, you should definitely go to Suomenlinna as Robert said. Nuuksio national park in Espoo is another option for summer or the first few weeks of autumn.