Computational thinking (CT) has become a necessary skill of students in the 21st century. Various learning approaches have been developed to foster CT among school students. However, these approaches predominantly rely on computer devices and internet connection and fail to promote advanced computer concepts necessary for programming. Therefore, this study developed an unplugged coding activity using flowblocks, the term is coined to represent modified Blockly based on flowcharts with user-friendly syntaxes, as a visual and programming tool, delivered in the form of game-based learning. The activity included a series of game missions to develop five programming concepts. The unplugged coding activity was implemented based on a pre and post intervention design with 160 secondary students who had no prior experience about programming. Statistical analyses showed that students' conceptual understanding of coding and CT increased significantly after participation. In addition, the perceptions of their ability to learn programming, namely self-efficacy, statistically grew in the posttest. It is therefore recommended for school teachers teaching basic programming and CT to consider using this offline, engaging and cost-effective approach as an alternative to computer-based methods of programming.