As a learner-centered pedagogy, project-based learning (PBL) is recognized to improve students' engagement and long-term learning effectiveness. The present PBL practices are based on instructor-defined topics and are assigned as a partial or full substitution of traditional assessments. Here, open-topic PBL as an opt-in activity is proposed and was applied to the teaching settings of engineering mechanics. The goal is to promote students' learning outcomes while shaping an inclusive learning environment. The intervention was introduced to the sophomore course Statics and Dynamics as an extra-credit group assignment complementing traditional exams. Participation was voluntary, and the project topics were student-defined based on real-life examples. The learning outcomes were evaluated by comparing students' improvements in exam scores between two study periods of the same semester, with the intervention applied only to the second. Results show that the intervention was successful overall and confirmed the positive influence of open-topic PBL on students' exam scores. The intervention's effectiveness was found to be gender-dependent; female students exhibited a higher participation rate and greater improvement on exams than their male counterparts. Moreover, lower-performing students before the intervention displayed a significant improvement after participation and contributed to a noticeable reduction in the class standard deviation. Keeping topics open and participation optional thus is recommended for PBL to account for students' individual uniqueness and to offer the flexibility to strategize every student's own learning approach suited to their unique situation.