Suomenlinna is built on an island off the coast of Helsinki, a short 10-minute boat ride from Market Square. The island retains its walls, fortifications, and docks; the old buildings are painted a pale yellow, and the green-domed church stands out. In summer, the grass is lush and green, and locals bring blankets for picnics. You can stroll along the walking trails, enjoying views of the sea and the skyline of the city across the water. There's a submarine you can go inside; it's cramped but quite interesting. There are no cars on the island, so a leisurely half-day walk is perfect.