Play and playfulness compose an essential part of our lives as human beings. From childhood to adultness, playfulness is often associated with remarkable positive experiences related to fun, pleasure, intimate social activities, imagination, and creativity. Perhaps not surprisingly, playfulness has been recurrently used in NIME designs as a strategy to engage people, often non-expert, in short term musical activities. Yet, designing for playfulness remains a challenging task, as little knowledge is available for designers to support their decisions. To address this issue, we follow a design rationale approach using the context of Live Looping (LL) as a case study. We start by surveying 101 LL tools, summarizing our analysis into a new design space. We then use this design space to discuss potential guidelines to address playfulness in a design process. These guidelines are implemented and discussed in a new LL tool—called the "Voice Reaping Machine". Finally, we contrast our guidelines with previous works in the literature.