My visit to Finland was a weekend trip to Helsinki during the white nights. At mid summer, because of the high latitude the sun only just dips below the horizon (and not for that long!).
There are many things to see in Helsinki - but it's also a great city to just wander around in.
Just near the harbour there is the Orthodox Uspensky Cathedral, complete with the golden 'onion' domes, and inside there are many Icons - they even allow you to take photos of the interior!
About a half an hour away from the city is the island fortress of Suomenlinna, it's worth a visit, but I must admit the day I went, there wasn't a great deal going on, and some of the attractions were closed. However I did get aboard the World War II U-Boat "Vesikko" - and it's a lot smaller that you'd expect!