Young people spend a large part of their day sedentary, both at school and at home. The aim of the PlayFit project is to persuade teenagers to lead a more active lifestyle by using digital as well as non-digital games and play. In this position paper, we describe in detail the three key principles of our vision concerning the design of game-based interventions for stimulating physical activity: playful persuasion, ambient action and play profiles. In our vision teenagers take part in playful activities and games throughout the day. In these activities, casual action is inherent to the fun experience, thus reducing teenagers' sedentary behavior. Relevant information about their activities and preferences is stored in a personal play profile, which affects the games they play and through which they can communicate to their peers. We illustrate this vision by means of several innovative game concepts.