A short ferry ride from Helsinki will take you to the UNESCO fortified island of Suomenlinna. The defenses were started by the Swedish government in 1947 however the island was captured by the Russians 53 years later making their effectiveness questionable. There are however plenty of tunnels to explore, bring a small flashlight.
It is rumored to be a good place to listen to the ice crack and sing as the thaw in the Baltic Sea starts. The shift and flow of the melting ice is definitely transfixing, especially as the Tallin ferry passes. Seems to be a Pokémon hotspot, plenty of rock and