If you do one thing in Helsinki, make it a trip to Suomenlinna. I may be slightly biased because I love exploring more than just about anything, but seriously, go to Suomenlinna!
Suomenlinna began as a Swedish naval base in 1748. It’s actually an island and only accessible by boat (in your case a ferry or a water bus). You board the ferry at Market Square, it’s very easy to find. The ride only takes about 15 minutes and there are multiple ferries back and forth every hour.
The island is open to visitors year round, in fact, there are about 800 people who live on Suomenlinna. There are a few restaurants and museums that you can visit. Unfortunately, nothing was open when we visited, so our visit centered around exploring the quiet streets.
There is so much to see on Suomenlinna! There are ship yards and narrow cobblestone streets. There are also fortified walls with dark tunnels inside. I suggest bringing a flashlight or some form of light (we used our phones) if you plan on entering the tunnels. They are extremely dark and a lot of them were full of water so they can become a bit of an obstacle course! It was an eerie experience exploring the maze of tunnels. I felt like I was a ghost hunter on one of those TV shows.
If you’re interested in history or an adventure, check out Suomenlinna while you’re in Helsinki!