Guided by the methodology of design research and the notion of seamless learning we develop a mobile learning activity for an outdoor context, where groups of 12 year old students are asked to coordinate themselves physically in terms of given distances with respect to both given and peer-defined points. Our learning activity consists of three connected tasks of successively increasing complexity, implemented at separate occasions over a period of 6 months. By participating in the activity, the students are offered opportunities to experience geometrical constructions in full-sized space. Specifically, they are stimulated to make use of their orientation ability, which differs cognitively from the visualization ability which is more commonly used to solve similar tasks in school. The outdoor explorations, the use of mobile technologies, and the distribution of the activity across time and locations, pose didactical as well as technological challenges which call for careful considerations regarding the design of the activity. In this paper, we account for the design process and its pedagogical grounding in ancient mathematics and modern psychology. Furthermore, we suggest to systematically combining the theory of instrumental genesis together with scenario-based design, within the methodological framework of design research, to guide the development of seamless mobile learning activities which provide a learning progression over time