If you are in Finland and have a little extra time, I recommend checking out Turku in addition to Helsinki. It is possible to take ferries to Turku from Stockholm, so that’s always an option… Turku is about 3 hours by train from Helsinki. It’s got a 400 year old castle that is now a museum and a 400 year old cathedral in the downtown area that is absolutely beautiful. I spent a year there studying as an undergrad and I have a lot of fond memories of the place.