This study presents the design rationale and prototyping process of GeoQuest, a mobile educational game designed to foster curiosity and promote lifelong learning in geography among adult learners. Grounded in game-based learning and design thinking, the project explores how user-centered design methods can inform the development of effective educational games. GeoQuest integrates map-based exploration, challenge-based quizzes, and incentive-driven mechanics to stimulate geographic thinking and self-regulated learning. The study follows the design thinking process, including empathizing, defining, ideating, prototyping, and testing, supported by simulated interviews, persona development, rapid ideation, and iterative prototyping. The primary objective is to investigate how design methodologies can be applied to create engaging and educationally aligned game systems for adult learners. The resulting contribution is a high-fidelity prototype, supported by documented artifacts and usability planning, which illustrates a replicable approach to serious game design. This work provides design implications for creating learning environments that support spatial reasoning, curiosity, and lifelong learning through game-based experiences.