In response to growing concerns about overtourism and the need for sustainable heritage tourism, this study investigates how external environmental stimuli affect tourists’ perceived value and behavioral intentions in historic urban environments. Using the Stimulus–Organism–Response (SOR) model as the theoretical framework, and based on 275 valid questionnaires collected from five major sites in the Historic Center of Macau, this study explores the direct and indirect relationships among four types of environmental stimuli (physical factors, social activities, environmental atmosphere, and information and services), perceived value, and behavioral intentions. The results of structural equation modeling (SEM) indicate that all four stimuli significantly influence perceived value and behavioral intentions. Among them, physical factors exert the strongest influence on perceived value (β = 0.291, p < 0.001), while social activities are the most influential predictor of behavioral intentions (β = 0.225, p < 0.01). Perceived value plays a significant mediating role in all relationships, with the largest mediation effect found in the path from physical factors to behavioral intentions (27.99%), followed by environmental atmosphere (24.80%), information and services (22.62%), and social activities (11.66%). These findings validate the applicability of the SOR model in heritage tourism contexts and highlight the central role of perceived value in shaping tourist behavior. Theoretically, this study advances our understanding of how multidimensional environmental stimuli contribute to value-based decision-making in tourism. Practically, it provides actionable insights for urban planners and heritage managers to design environments that promote deeper engagement and foster sustainable tourist behavior in high-density historic destinations like Macau.