Suomenlinna is an old sea fortress spread across 6 tiny islands in Helsinki, Finland. A quick 15 minutes ferry ride from the heart of the city, it's an ideal afternoon excursion, particularly on a warm summer day. You can spend hours simply wandering the islands, stopping in the local galleries or taking a picnic break. If you're feeling a bit more adventurous, you can explore the fortress ruins and walk through the various connecting tunnels. LINK.
Runner Up(s): Primrose Hill, London