'ActiCiti' revolutionises engagement with the built environment through immersive mixed reality (MR) experiences. Using a personalised MR platform, users can transform their perception of the physical world by overlaying digital data, unlocking a new level of customisation and exploration. Blurring the lines between reality and the virtual world, ActiCiti shapes environments and digs into digital materiality. It explores the remarkable potential of AI-generated content to extend the performance of materials in digital overlays, enabling the attainment of object properties that surpass the limits of reality. Users become co-creators, fostering creativity, collaboration, and a deeper connection to urban spaces.