In this work we discuss Fun in Numbers, a software platform for implementing multiplayer games and interactive installations, that are based on the use of ad hoc mobile sensing devices. We utilize a detailed log of a three-day long public showcase as a basis to discuss the implementation issues related to a set of games and installations, which are examples of this unique category of applications, utilizing a blend of technologies. We discuss their fundamental concepts and features, also arguing that they have many aspects and potential uses. The architecture of the platform and implementation details are highlighted in this work, along with detailed descriptions of the protocols used. Our experiments shed light on a number of key issues, such as network scaling and real-time performance, and we provide experiments regarding cross-layer software issues. We additionally provide data showing that such games and installations can be efficiently supported by our platform, with as many as 50 concurrent players in the same physical space. These results are backed up by a user evaluation study from a large sample of 136 visitors, which shows that such applications can be seriously fun.