Scientists in Belgrade came up with the idea of “planting” large tanks of water and algae in places where trees can’t grow. The tanks are 10-50x more efficient than a normal tree for the space it takes up and is in general highly sustainable, even creating excellent fertilizer in the process. You can skip about halfway through the video for the actual information about them.
Yes ideal would be to remove cars completely from cities and use bikes + grassy trams to get around. And have every street lined with trees. In places where that hasn’t been implemented yet, there may be a place for an algae tank.