Based on your search query, I recommend using Scratch to create an online sustainable city. This platform is not only free but also encourages creativity and systematic reasoning, which are essential skills for designing a sustainable city.

With Scratch, students can learn coding through the creation of interactive stories, games, and animations. They can use these features to create a virtual model of a sustainable city, incorporating elements such as renewable energy sources, green spaces, and efficient transportation systems.

The block-based programming language of Scratch is easy to use, allowing students to focus on the logic and problem-solving aspects of building a sustainable city. They can experiment with different solutions, see the results in real-time, and refine their designs based on feedback.

Moreover, Scratch promotes collaboration, enabling students to work together on their sustainable city project. They can share their projects with others, learn from each other's ideas, and collaborate to create a more comprehensive and effective design.

Finally, Scratch is accessible both online and offline, ensuring that all students can participate in the project, regardless of their internet access.

In conclusion, Scratch provides a fun, engaging, and educational platform for students to learn about sustainability and apply their knowledge in a practical and creative way.