Most American girls are not meeting physical activity guidelines, placing them at risk for adverse health consequences. In response, our team developed Frolic, an evidence-based mobile application to address this problem through play. Frolic uses a socio-ecological framework and behavioral design principles to address barriers and social norms that are specific to girls. Parents are leveraged as important sources of support and motivation, with key design elements to help girls initiate play, adapt to the space, and play independently. The current paper contributes design principles for creating socio-ecological activity games for girls. In addition, we contribute the design of Frolic itself, a game constructed according to these guidelines, and outline future intervention studies with the app itself. Finally, we provide the perspectives of professional game designers and health experts on our work and interpretative insights for game designers and researchers who hope to work in this space.