This paper explores a bottom-up way to speculate towards future smart cities by inviting residents of Copenhagen, Denmark to participate in sensing activities. It illustrates how the idea of “design things” could support bottom-up citizen participation in a smart city. It uses a research through design approach, deploying a wearable air quality sensor to three Copenhageners. By investigating citizens’ perception of the city through this prototype, we illustrate a possible path for engagement in the development of future smart city technologies that offer a greater sense of influence and relevance for residents. Further, citizen participation in sensing activities provides a route to different understandings of smart cities: as a place for people and participation instead of for data and rationalisation.