This paper describes high school STEM lessons created by three undergraduate Mechanical Engineering students. The students participated in a classroom outreach program called Computer Science Investigations (CSI: Cincinnati) as part of the Mentoring for Connections to Computing effort funded by the National Science Foundation's Broadening Participation in Computing program. The CSI program performs computing-themed pre-college outreach with a team of undergraduates who deliver original lessons with hands-on activities and interact with high school students in their classroom. This paper describes three lessons: (1) pipe design layout based on CAD drawings for an industrial bakery kitchen, (2) computer vision systems using optical illusions and cupcake decorating, and (3) manufacturing of plastic pellets using the creation of pixel cookies as a hands-on classroom activity. The undergraduate mentors leverage their industrial and academic experiences to create the lessons and act as role models for college and professional success. Evaluation of the activities includes a mapping to academic content standards, student interest surveys, and mentors' reflections on their experiences.