Street art and political activism have a rich history of shaping urban landscapes. Our work explores the processes by which public artists and political activists contribute to public spaces, introducing opportunities for HCI researchers to engage with the people who shape the aesthetic feel of our cities. We present WallBots-autonomous, wall-crawling robots as a research probe for public expression across a wide range of surfaces and hard-to-reach places, including bus stops, whiteboards, streetpoles, trashcans, moving vehicles and building walls. We evaluate WallBots as a low-cost DIY authoring tool for public artists and activists. Our study of six individuals who extensively contribute to public spaces offers insights into the materials and practices behind grassroots public expression. We then leverage feedback from participants, among them a graffiti artist, light painter, political activists, and street musician, to evaluate interaction techniques for manipulating WallBots as a medium for public expression across a range of surfaces. Our findings expose a research space for technological interventions in the context of street art, and we conclude with design insights for magnetic kinetic systems as an approach for supporting engagement, expression and creativity in public spaces.