GeoFORCE and STEMFORCE are two summer enrichment programs offered by the University of Texas' Jackson School of Geoscience. These two programs aim to increase the number of high school students from diverse backgrounds who pursue a degree in the geosciences or STEM-related fields. To better align to the rigor of STEM-related coursework, we have developed and implemented a new capstone experience, first piloted in 2017 with 120 rising high school seniors in the GeoFORCE and STEMFORCE cohorts. The curriculum developed for this approach follows the STAR Legacy Cycle, a challenge-based approach that drives learning by integrating the scientific process and a products-based assessment. The products students are expected to create include a conference-style poster, a 12-minute technical presentation, and a 4-minute "lightning-talk" geared towards a general audience. Teams of 4-6 students assume the roles of geoscientists to solve the "Central Texas Challenge", a real-world problem which challenges students to consider the real-world implications of research and innovation. Students solve the challenge by combining geologic field observations and independent research, using technology and engaging activities that create an awareness of geoscience careers. The instructional team consists of two expert geoscientists (faculty and PhD students), a master teacher, and three undergraduate students to facilitate the learning experiences in the field and the classroom. Educational research and post-trip student evaluations indicate that the STAR Legacy Cycle-based curriculum is an improvement over previously applied GeoFORCE curriculum. Students initiallly struggle to adapt to self-directed learning, but evaluated the curriculum to be more enjoyable due to the rigor and content of the deliverables. Students report that this approach encourages college-readiness skills that need to be practiced: time management, note-taking, group assignments, professional presentations, and written/visual communication. Finally, students reported increased motivation to take on new challenges and seek new opportunities given their success through this experience.