Outdoor, multi-player games involving social interaction and physical activity are an emerging class of applications particularly interesting for children, for whom the attraction and the health and developmental benefits are clear cut. Implementing and prototyping such games present non-trivial technical challenges to interaction and game designers; this hampers iterative prototyping and testing cycles that are core to user-centred design and game development processes. This insight has motivated the development of RaPIDO (Rapid prototyping of Physical Interaction Design for Outdoor games), a prototyping platform for physical computing, targeting interaction designers with limited electronics or software skills. RaPIDO has been evaluated in a user test, evaluating RaPIDOs software library, and in a case study involving two designers who used it to develop outdoor games for children. We illustrate how RaPIDO enabled broader exploration of the design space and faster iterations than would otherwise be possible, allowing designers to focus on the core game concepts rather than complex and low-level engineering issues.