Headstrong is a cutting-edge mobile app designed in partnership with the New Zealand Ministry of Health (Te Whatu Ora) and The University of Auckland to promote mental and physical wellbeing among young people. With its evidence-based, culturally responsive strategies, including CBT, positive psychology, mindfulness, interpersonal skills, and substance harm-minimization, Headstrong empowers users to take control of their wellbeing and make positive change. Ongoing collaboration with our target audience identified a need to better support users with developing and maintaining a healthy relationship with physical activity; with only a third of this age group meeting the recommended 60 minutes per day. The new physical activity experience was co-designed, and later piloted with school-aged children before being further refined based on user feedback and key insights. The result is an app that promotes a healthy relationship with exercise through a user-friendly, visually appealing interface and a range of engaging features. The app's goal planning and tracking, mood monitoring, and interactive activity modules make physical activity fun and achievable. The personalised experience, through a guided journal and AI-driven virtual coach, provides motivation and support to help users stick to their physical activity goals and understand the connection between physical activity and emotional well-being. In conclusion, Headstrong is a well-researched, innovative, and user-centred mobile application motivating young people to stay active and maintain a healthy relationship with physical exercise.