Mobile games have the potential to increase physical activity (PA). However, not all games can capture and sustain interest in PA, as it is challenging to maintain user engagement once the initial novelty diminishes. This research explores how augmented reality (AR) and idle game design, combined with persuasive strategies, can reduce sedentary behavior. We designed, developed and evaluated PetBuddy, a mobile health game aimed at promoting PA. Sixty-five young adults (between 18–35 years old) played the game for 10 days and completed a questionnaire about their experience. This is followed by an interview of 17 participants. Results revealed significant increases in PA with users meeting the World Health Organization’s recommended activity levels and adopting healthier behavioral changes. Participants reported that the persuasive strategies implemented as game features (particularly competition) and the game experience motivated them to participate in PA. The findings from this research provide a deeper understanding of how mobile games can be designed to encourage PA.