The XVII century island fortress of Suomenlinna is an open air museum and a small museum of crafts placed in the small village. The last one mentioned for sure has entrance fees, but you are free to visit the island without paying anything.
Just mind the seasons! Suomenlinna can be extremely cold and windy: if you choose a February day you could be going through a nightmare-journey! In my case, maybe because of bad luck or not thinking too smartly, my visits have always been unbearably cold.
The island could give people the impression of being very “dark” or “creepy” as there is always a feeling you have not visited it all, or missed something important, some secret galleries etc.
Indeed, the island is full of dark galleries with muddy grounds to walk on: that’s what makes it scary to me. But many people told me that kids actually love it and see it as a place for their adventures.
I have been actually once to Suomenlinna in May, on a sunny day. It was still very windy but…what can you do about it, we are in Finland! That day the thing I liked the most was to sit on a cliff facing a little water corridor and waiting for the boat to Sweden to pass by there. I know that during summer the island is quite crowded on the beach area. Did not try it yet, but I must.
The island is reachable by ferry from Kauppatori, and it is accessible with a public transport ticket.