The Stockholm archipelago. Unique experience during winter and summer. If you don't have your own boat, buy a ferry week-pass, so you can do some island-hopping. But it is just not beautiful - don't miss the nightlife on Sandhamn and Utö.
Don't miss a night cruise (dinner and breakfast included) through the archipelago between Stockholm and Helsinki. The islands, with their quaint homes and picturesque harbors, will etch a place in your soul.