There is a need to reframe teachers' roles from content area experts to that of learning experience designers because of 21st century teaching challenges. As learning experience designers, teachers help students develop 21st century competencies via guided cognitive and social participation in designed learning activities such as games and gamified activities. In this qualitative case study, we explore teachers' design thinking in designing a lesson plan that involves the use of a card game designed for complex system understanding. Six teachers' thoughts about learning activity design are unpacked via the following activities: playing a card game, crafting a game-based lesson plan, and reporting their design thinking via semi-structural interviews. We discuss similarities in the teachers' views on game-based learning and the structure of their designed lessons, as well as implications of the study. This baseline study helps us map out how teachers think about learning experience design. Such understanding is critical for developing teachers as designers.