Serious virtual worlds (SVWs) represent a great opportunity for learning and should embed a lot of high-quality contextualised information so that the player can take the most from his or her exploration of the environment. This requires defining new methodologies and tools for effective production. We have investigated this issue in implementing a couple of SVWs and abstracted a conceptual framework relying on the task-based learning pedagogical theory. The model defines games set in realistic SVWs enriched with embedded educational tasks. Tasks are simple trial activities that embody units of knowledge. The player can discover them in his or her SVW exploration and interact with them in order to construct meaning, build lasting memories and deepen understanding of the featured item(s). The model involves pedagogical task annotation--which allows decoupling the tasks, which can be reused in different VWs from the definition of their delivery strategy in the context of a specific VW, which is specified by the VW designer and automatically managed by the run-time engine. This approach simplifies the authoring work. The visual Creative Toolkit (CT) we have developed is being used for producing contents for the 15 reconstructed cultural cities that will be featured in the first version of the Travel in Europe VW. The paper described the overall framework and the details of the CT. It also presented an implemented example, briefly discussing the methodology we have followed for developing cultural heritage content.