Out-of-school making and engineering programmes that are frequently positioned as playful have increased dramatically in recent years -- but how appropriate is the framing of play for engagement in these informal STEM (science, technology, engineering and mathematics) spaces? Drawing on data from two research sites located in the UK, including observations of making and engineering activities with children aged 5-13 years, and interviews with nine key informants, we identify that play has three key affordances namely: (1) play can provide structure, (2) play is considered to be synonymous with open-ended science inquiry, and (3) play can enable gender inclusive STEM spaces through promoting free-choice. We also note that overly simplistic framings of play may limit recognition by both adults and children of the educational value of these spaces and deny a fuller understanding of the opportunities that such spaces afford in providing children with more opportunities to engage with STEM, particularly engineering. We suggest that play has an important pedagogical role in informal STEM activities, including making, when it is grounded in free-choice exploration and imagination. We argue for continued discussion and reflection upon both the value of play as a pedagogical vehicle and its affordances for enhancing youth engagement in STEM spaces.