This paper investigates one specific cultural ecosystem service: outdoor recreation. We present a methodology to map the collective social value of outdoor recreation and identify the substitutability among nature sites within a specific spatial context. This methodology is applied to the province of Antwerp, Belgium. We propose an indicator of substitutability among nature areas, contrasting unique but poorly substitutable sites (hot spots) with highly substitutable sites (cold spots). Using a combination of survey information, public participation GIS (PPGIS) and kernel density mapping, we produce density surfaces representing the distribution of the collective social value attributed to outdoor recreation. We also compute Getis-Ord Gi* spatial statistics to identify local outdoor recreation clusters. In addition, we explore how recreational behaviour affects substitutability. Our results suggest a duality between the social value of outdoor recreation and the level of substitutability among nature sites. Highly substitutable sites tend to be found near areas of higher population density, which are as well sites of higher social value. Individual-specific parameters such as the type of recreational activity appear to substantially modify substitutability patterns among nature sites. We conclude by discussing the methodological and policy-related implications of this research.