School playgrounds are important physical activity (PA) environments for children, yet only a small number of children reaches the target of 40% of moderate-to-vigorous PA time during recess. The aim of this study was to explore children’s perspectives (i.e., child-identified determinants) of activity-friendly school playgrounds. We conducted participatory research with children as co-researchers, framed as a project to give children the opportunity to discuss their views and ideas about their school playgrounds. At three schools, six children (9–12 years old) met over five to seven group meetings. Data analysis included children’s conclusions obtained during the project and the researcher’s analysis of written reports of all meetings. Children indicated a strong desire for fun and active play, with physical playground characteristics and safety, rules and supervision, peer-interactions, and variation in equipment/games as important determinants. Our results indicate that improving activity-friendliness of playgrounds requires an integrated and multi-faceted approach. It also indicates that children, as primary users, are able to identify barriers for active play that are easily overlooked, unknown or differently perceived by adults. Hence, we believe that structural involvement of children in designing, developing and improving playgrounds may increase children’s’ active play and consequently PA levels during recess.