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 - it's a great city to just wander around in, there's so much to see.
I visited the Temppeliahkio church which is hacked out of the living rock, it looks a bit like a James Bond baddies headquarters! A stroll around the Eira district to the south of the centre is worthwhile if you are interested in architecture. On a sunny day the nearby Kaivopuisto park gives some great views back across the city.
The Senate square has the cathedral (Tuomiokirkko), and is surrounded by very Russian looking buildings.