Make all city street landscaping (food forest) both edible and sustainable by capturing rainwater.
A "food forest" is a combination of edible landscaping form plants available to the public, free. By using city/street landscaping islands for this, rain water can be captured and diverted, the plants provide both shade and food, water use is minimized and the whole city becomes foot traffic friendly and cleaner air. Tucson is doing this now -- http://www.upworthy.com/can-somebody-just-give-this-guy-a-million-bucks-already?g=2