What are the low hanging ideas and concepts to make cities more sustainable?

With cities worldwide facing challenges on environment and energy, they are desperately seeking solutions that can make them more sustainable in a short time. We need to be smart in identifying easy, low hanging ideas that have the potential to transition cities to sustainability quickly. What could such concepts and solutions be?

How will it have a positive impact?

Who will it have a positive impact on?

Urban Community

Developed Countries