The stronghold of Smelina is often called Finland's Fort. You can understand it when you talk to the Finns. This fortress is not in the centre of the city. It's going to take a boat to an island. Fortunately, one-day transport tickets are available in Helsinki, and no extra tickets are required. In summer, there are boats to the King's Gate, so they don't have to go back in the same way. It's better than in winter.
Suomenlinna is a treasure trove. There is enormous amount of photogenic places - old ships, forts, cannons. Dry dock area is bustling with activity twice a year when large old wooden ships are taken in and out. In the summer time there are several cafe's open, other times only couple near ferry harbour. Suomenlinna has swimming beaches, museums, brewery restaurant. In summertime the place is beautiful but can be crowded in most popular places. Even after decades of wandering - still possible to find new nooks and crannies - peaceful places.
This island fortress is accessed by a short ferry ride from the city harbour and is a brilliant day out. The island is big enough to spend several hours on and the historical features are most interesting. There are some places to eat but I recommend taking food and enjoying a picnic if it's sunny. Check out the old church as well in the centre of the island.
I was here at the beginning of March when the weather was freezing, but it was still a fantastic place to visit. There were very few other visitors, which was a plus, but many of the restaurants and cafes were closed for the season, which was not! It must be truly amazing when the weather is warm, though I'm sure it is much more crowded.
The Fortress of Suomenlinna is more about the foreign powers that were battling over Finland than about Finland itself. Finland gained control of the fort around the time of their independence in the early 20th century. Highlight is probably the dry dock, which is huge, and the video at the Suomenlinna museum.