Based on your query, I would recommend using Scratch to create a sustainable city. This platform is not only free but also encourages creativity and systematic reasoning, which are essential skills when planning and building a city.

With Scratch, students can use the block-based programming language to create interactive stories, games, and animations. In this case, they could design a game or simulation that represents a sustainable city. They could incorporate elements such as renewable energy sources, waste management systems, green spaces, and public transportation.

The platform also promotes collaboration, allowing students to work together on their city project. They can share their projects with others, encouraging feedback and idea exchange.

Moreover, Scratch is accessible both online and offline, making it a versatile tool for different learning environments.

Remember, while Scratch is primarily a coding platform, it can be integrated into various subject areas. In this case, it could be used to teach concepts related to urban planning, environmental science, and sustainability.

I hope this helps! Let me know if you have any other questions.