Student learning outside the formal classroom is inextricably linked to informal learning environments. In many countries, most activities that employ informal learning prioritize the integration of science, technology, engineering, and mathematics (STEM) disciplines and have shown a positive impact on increasing students' interest, self-efficacy, and awareness of the STEM field. Thus, this study aims to systematically review the activities reported in the relevant studies focusing on informal STEM learning for K-12. High-index journals published under SCOPUS and Web of Science databases were utilized using a predetermined search strategy and retrieved two research team members' screened articles. Only empirical studies containing the terms "STEM education", "summer camp", and "informal learning" in the title, abstract, and keyword were included. Data were coded and organized into a matrix that was qualitatively assessed and categorized into themes. Based on the 25 studies reviewed, it was found that the integration of STEM disciplines in informal learning is notably varied. The characteristics of the STEM activities can be organized into seven themes: inquiry, focus on problems, design, cooperative learning, student-centred, hands-on, and 21st-century skills. Practice recommendations include a quality curriculum that aligns with informal STEM learning needs.