One of the world's largest sea fortresses, the 18th-century fort on Suomenlinna is a 15-minute ferry ride from Helsinki's Market Square (a mini-cruise that has lovely views of the city as a bonus attraction).
Description
Once here, you could easily fill a day with its sights and activities. The impressive fortifications, now a UNESCO World Heritage Site, were built in 1847 by the Swedes (Finland was Swedish territory then) to scare off the Russians; they weren't scared and later captured both the fort and Finland.
Begin with the audio-visual experience in the visitor center (it's in English) for a lively history, then explore its ramparts, tunnels, and museums and walk the trails around the beautiful island. Or sign up here for a guided walk to learn more about the fort and its various attractions. Among these are the 250-ton Vesikko submarine, used by the Finnish Navy from 1936 until the end of World War II.
The Ehrensvärd Museum illustrates the earliest Swedish period, and the Doll and Toy Museum displays dolls, dollhouses, and toys in an old Russian villa. Various buildings house studios and shops of glassblowers, potters, and other craftsmen, and in the summer, you can stay for evening dance and musical performances of the Suomenlinna Summer Theatre.
Official site: https://www.suomenlinna.fi/en