Everyday life in cities isn't purely rational. This is particularly true of activities in public spaces, where the social context is complex and relatively unstructured. This paper focuses on a diversity of playful activities observed in one kind of urban space - public staircases. These observations illustrate how the design and location of urban space frames opportunities for playful social behavior. Play opens up new possibilities of social practice, and in doing so expands the possible meanings and functions of urban space.