Serious games present a promising opportunity for learning, but the genre still lacks methodologies and tools for efficient and low-cost production, particularly for teacher and domain experts. This article gives an authoring framework that aims to provide structured support, from content design to final implementation. In particular, we have abstracted a conceptual model—the SandBox Serious Game - which relies on a generalization of task-based learning theory. The model invites players to perform cognitive tasks contextually while exploring information-rich virtual environments. We consider it particularly suited for cultural heritage entertainment applications. The model defines games that are set in realistic virtual worlds enriched with embedded educational tasks, which we have implemented as minigames. This approach simplifies the authoring work, which can easily be supported by visual authoring tools for ontology-based urban 3D modeling and implementation tasks, thus allowing an approach similar to the mind-maps concept. We propose a top-down methodology for content preparation, starting from a city-level analysis down to the single points of interest and associated tasks, which are instances of simple predefined minigame/quiz typologies. We provide examples and discuss criteria for selecting task typologies according to the authors' cognitive targets. Finally, we discuss the results of a user test, which took place in a lab, aimed at verifying the acquisition of cultural heritage knowledge in a pleasant and engaging way. Games appear particularly suited for supporting the study of images, especially of iconography. Compared to reading text, a game forces the player to focus more strongly on problems, which favors knowledge acquisition and retention. Learning complex concepts requires an investigative attitude, which can be spurred by well-designed games. Good design involves usability, graphic appeal, appropriate content, and the presence of connections which a player must discover in the content. Players should be asked to pay attention to and reason about their whole game activity - including the relationships between the game content, the brief introduction, and concluding texts. More comprehensive tests are needed to better investigate the educational effectiveness—however, the first results are promising, especially in terms of user motivation and creation of new opportunities for learning about CH.