Suomenlinna, Helsinki, Finland

Suomenlinna, the ‘fortress of Finland’, straddles a cluster of car-free islands connected by bridges. The Unesco World Heritage site was originally built by the Swedes as Sveaborg in the mid-18th century. Several museums, former bunkers and fortress walls, as well as Finland’s only remaining WWII submarine, are fascinating to explore; its tourist office has info. Cafes and picnic spots are plentiful.