This paper describes the design and a preliminary evaluation of the AI Workbench, an interactive tool for fostering AI literacy for secondary school students by enabling them to prototype user-facing AI applications without programming. It is based on a competence model for AI literacy that combines technical exploration and ethical reflection, in order to support a critical understanding while also encouraging responsible engagement with AI systems. The workbench focuses on building simple image recognition applications, but goes beyond related tools by providing customizable multimedia outputs and a dual view interface allowing students to explore the perspectives of both developers and end-users. In this way, it highlights the problem of the opacity of AI and its real-world implications, encouraging students to critically examine issues such as transparency, fairness, and ethical considerations. The results of a preliminary evaluation in two workshops with 57 secondary school students provided positive indications of the effectiveness of the tool to support experiential learning about AI. Participants reported a better understanding of both AI mechanics and basic ethical issues, as well as high user satisfaction. Further development will include scenario-based simulations, such as autonomous driving test environments, to better contextualize technical and ethical decisions.