It has been almost 4 years since I visited Finland, and one of the best souvenirs I have from that trip was the day trip I made to Suomenlinna on a nice summer day. It's a very interesting place; according to wikitravel, "Gibraltar of the North was once the greatest sea fortress in the Baltic, built by the Swedish in the mid-1700s." You can take a boat that will bring you to this place, enjoy the day, and then come back to Helsinki. Totally worth it!