Social cognitive career theory posits that career interests and decisions are influenced by individual, contextual, and behavioural factors. This research aims to assess the impact of Science, Technology, Engineering, and Mathematics (STEM) activities on students' interest and aspirations in STEM careers. Additionally, the study aims to explore the critical factors that might shape students' STEM career aspirations. The research used a quantitative approach, with pre- and post-test measures for a cohort of 191 Portuguese secondary school students, using the Career Interest Questionnaire (CIQ). The results showed that STEM activities increased the intentions of non-STEM students to pursue educational opportunities that could potentially lead to a scientific career. Males, in contrast to females, showed a significant increase in their intentions to pursue educational opportunities that could eventually lead to a scientific career. Furthermore, gender along with the personal goals and positive perceptions of STEM careers were identified as significant predictors of the students' STEM career aspirations. These results highlight the critical role of STEM education in developing and sustaining students' interest in STEM careers. Integrated STEM activities should be developed in the early stages of education to enhance students' confidence in STEM and mitigate the gender gap.