The Suomenlinna Seafortress is on a couple of islands just outside Helsinki, so it will take a while going there and coming back. You’ll probably want to reserve more than two hours for the whole thing so I recommend that you plan eating there as well. There are a couple of restaurants there, but I recommend that you do a picnic instead. There are excellent spots to do this all over the islands, but a really good one is the southern end of Kustaanmiekka. And remember, DO NOT LITTER! Ok? ;-)