This study explores the characteristics of after-school program activities at a charter school in the Southeast US highlighting students' experiences with and gains from these after-school program activities. A qualitative case study design was employed to understand students' views and opinions regarding the activities and their learning trajectories. Study data were collected through formal and informal observations, one-on-one semistructured interviews, and field notes. The study's findings indicated that such activities emphasize open-ended and collaborative scientific investigations in Science, Technology, Engineering, and Mathematics (STEM) fields and provided an arena for students to demonstrate various uses of 21st century skills. We have described and explained: (a) the importance of collaborative learning groups, (b) the popularity of after-school program activities, (c) interest in STEM fields, and (d) activities' contribution to developing 21st century skills. These findings show that STEM related activities have the potential to promote collaborative learning and inquiry as well as to contribute to the development of 21st century skills. These findings have also been discussed in light of how STEM related after-school program activities support students' learning.