One thing is sure, if you have the chance to visit Helsinki city for at least 2 days, Make sure to add the Suomenlinna Fort on your to do list.
One thing that attracted us as travellers and discoverers was that it's part of the UNESCO World Heritage list. On their website you'll find lots of information. For example, how it was built by Sweden, on a group of island, in the second half of the 18th century and located at the entrance of Helsinki’s harbour. It's easily accessible by boats and waterbuses and easy to spend one entire day discovering all secrets places around.
We recommend that you dress accordingly. Depending of the day and time of the year you go, prepare for the worst (wind and rain) just in case. Keep in mind that it's a little way from the shore. We were there at the end of May/2017 and it was pretty cold. We got lucky that didn’t rain all that much and we could walk around all island.
On the island, you will find restaurants, small shopping stores and a brewery if you want to take a break. On warmer days, it could be a good idea to bring your own food and find a nice area for a picnic. There are many beautiful locations, in front of the sea or between the historical buildings.
There is a Church built by Russian troops, a museum and other popular sights such as the “King’s Gate” drawbridge.
We preferred to explore the island and the nature walking on the trails that lead us to see the shoreline around the fort. Really great to discover small beaches and picturesque paths!