In this position paper we describe our vision on designing playful interactions to persuade people to be physically active in public spaces. Social embeddedness and playful interaction are the core elements of this vision. We illustrate how our design vision is incorporated into innovative concepts that stimulate physical activity for various user groups and in various use contexts, and present some general findings on the basis of these cases. New technologies such as mobile networks and social media provide new opportunities for creating location-independent solutions that support groups of people to motivate each other to be physically active by creating challenges for each other. Designing playful solutions for public spaces asks for low-threshold solutions that support easy stepping in and stepping out solutions.