Integrated approaches to teaching science, technology, engineering, and mathematics (STEM) are increasingly being implemented in elementary and middle school classrooms, and despite a variety of conceptions of integrated STEM, researchers agree that small group activities and teamwork play a central role in STEM learning. However, little is known about how students participate in the small group portions of integrated STEM curricular units. In this study, a microvideo ethnography framed within activity theory was used to examine small group interactions among sixth-grade students completing integrated STEM activities related to the properties of light. Students working in three different small groups (all-girl, all-boy, and mixed-gender) were included in the analysis. Findings highlight differences in the activity systems across activity type (science vs. engineering) and across small groups, with students focusing on different objectives for completing STEM activities, utilizing different tools as they sought to reach their objectives, and dividing labor differently. Findings from this study suggest that these students, and girls in particular, were less prepared to navigate open-ended engineering activities than highly structured science activities. Theoretical and practical implications for curriculum development and pedagogical strategies are discussed.