Self-monitoring technologies (such as heart rate monitors and activity trackers) that sense and collect physical activity data are becoming increasingly common and readily available. These devices typically represent the captured data using numbers and graphs that primarily appear on digital screens. More recently, representing data in a physical form such as 3D printed physical artifacts is gaining currency within HCI, owing to the engagement opportunities that come with physical representations. However, there exists a limited understanding of how to design such physical representations of personal data. To contribute to this understanding, we present a set of ten design themes, developed from the analysis of two independently designed systems that construct 3D printed physical artifacts from physical activity data. Each design theme describes a unique design feature that designers could incorporate in their design to make physical representations more engaging and playful. We envisage that our work would encourage and guide designers to think about different ways of supporting physical activity experiences.