Suomenlinna is an old sea fortress originally built by the ruling Swedish in the 1600s. It costs 5€ round-trip to get a ferry to the island it’s on. The day I visited Suomenlinna, the weather was warm and sunny. People were lying on the rocks and swimming in the sea. We walked from the point where the ferry first landed on the island to the King's Gate, where the king used to arrive from. On either side of the gate are inscriptions from the different Swedish kings that visited the fortress.
Suomenlinna is surprisingly large and very well maintained. There are several museums on the island that cost an extra 3€ or so to enter. There is also a youth hostel, which I didn't stay at, but am tempted to next time. However, I did go on a beautiful day, and I imagine it could get quite cold at Suomenlinna when it’s windy.