This study builds on the growing use of serious games in sustainability contexts by using principles of Causal Loop Diagrams in a game-based format to strengthen systems thinking and co-create actionable measures in energy transitions. Our game-based approach unfolds over three phases. First, participants engage in card arrangement and system mapping to co-construct a shared understanding of the socio-technical energy system using system cards. Second, they establish connections and feedback loops of these system cards. Third, participants identify measures for change by proposing interventions aligned with local energy goals and visions. Two case studies demonstrate the applicability of this approach in the municipality of Saint-Prex in Switzerland and the North East region of England. In Saint-Prex, participants emphasised behavioural change and trust in local governance, highlighting a feedback loop where solar PV adoption boosted social acceptance and perceived economic viability. In North East England, participants surfaced barriers such as policy fragmentation and infrastructure gaps, while identifying reinforcing feedback loops between EV manufacturing and battery production that revealed industrial synergies. Across both cases, the game-based approach enabled participants to visualise system dynamics, co-create strategies, and build shared mental models. The workshops were evaluated positively, with participants reporting increased empowerment. Future research could examine how the co-developed measures might be effectively prioritised and temporally sequenced to form coherent transition pathways.