Fort outside of Helsinki

https://www.suomenlinna.fi/en/

Suomenlinna Fortress is on an island a short but scenic ferry ride from Helsinki.

Suomenlinna is accessible only by water. A ferry service runs from the
Market Square to Suomenlinna throughout the year. In the summer
season, a water bus service to Suomenlinna is also available. The
guest harbour in the middle of Suomenlinna caters to visitors arriving
by their own boats.

The trip to Suomenlinna takes 15–20 minutes and offers magnificent
views of Helsinki and the surroundings from the sea. During the cold
winter months, the trip through the ice-covered waters is a unique
experience.

Helsinki is a compact walkable town. Walk about and check off the Rick Steves sights:
https://www.ricksteves.com/europe/finland/helsinki

Thanks Edgar, appreciate it.

I visited Helsinki on a day trip via ferry from Tallinn and very much enjoyed the 3 hours I spent on Suomenlinna--it was early Sept and the weather was perfect for exploring the island. I also enjoyed the market at Kauppatori esplanade (the Market Square referenced above)--interesting hand-crafted items, and as I'm a knitter I was quite happy to find a seller of hand-dyed wool. The rest of the day I spent in downtown Helsinki.

If you are interested in war history as part of your visit in Helsinki, there is the Tank Museum in Parola, get there by local train. There is also the Finnish-German military cemetery too located in Helsinki.