In this paper, we argue that a complexity-driven systemic perspective of urban life can be characterized through the consumption practices of people that interact within urban spaces, and that such a characterization can uncover relevant information for planners. We propose a granular analysis of commercial amenity networks within cities and adapt a measure of economic complexity to aptly reduce these networks. This yields the Amenity Complexity Index (ACI), which is the focus of this work. The ACI is interpretable in terms of consumption practices and of place characteristics, both from a macro city perspective and from a place-based granular perspective. The dynamic illustration of our complexity measure in Paris helps demonstrate ways in which the ACI can enrich our understanding of cities and of the transformative systemic challenges they face. Ultimately, this work proposes a measure and an accompanying interpretation of urban complexity based on commercial amenities that paves the way for novel analyses of the causes and effects of urban transformations, of urban policies, and of the wellbeing of dwellers.