Based on your search query, I recommend using Scratch for your online sustainable city project.

Scratch is a free platform that allows students to learn coding through the creation of interactive stories, games, and animations. This could be an excellent tool for your sustainable city project as students can create an interactive model of a sustainable city, demonstrating their understanding of sustainability concepts.

The platform encourages creativity, systematic reasoning, and teamwork, which are all crucial skills for a project like this. For instance, students can work together to design different aspects of the city, such as energy sources, transportation systems, and waste management solutions.

Moreover, Scratch provides resources like project ideas, tutorials, activity cards, and educator guides, which can be helpful in guiding your students through the project.

Remember, Scratch is a free platform, so there are no costs associated with using it for your project.

I hope this recommendation is helpful for your project. If you have any other requirements, feel free to ask!