After the success of New York's High Line, every forward thinking urban planner in America wants to turn their city's elevated railway into beautiful parks. But that's both derivative and often not practical for the environment. That said, there are many ways that cities can remodel derelict infrastructure, or repurpose city infrastructure that was poorly designed in the first place.
The American Society of Landscape Architects (ALSA) has compiled many of these ideas into a handy video about redesigning cities to create healthier and more user-friendly cities into one handy (though dryly narrated) video, with some amazing renderings of the lovely, nearly bucolic cities in which we may all live in the future.
One of the main undervalued (if still overused) assets of any city is parking spaces. By making fewer parking spaces, you can deincentivize driving, which makes city life generally more healthy. At the same time, you can take the land otherwise used simply for storing cars and turn it into public spaces. It's something that's been done on a temporary basis by Park(ing) Day, but if cities took it from a yearly culture jam to a permanent policy, it could quickly alter the urban landscape.
Interstate highways—once drive through the centers of cities as an emblem of progress—are now fairly universally acknowledged to do little to improve the lives of the people who must live and work under them. While projects like Boston's Big Dig have removed the highway from the city and replaced it with green space, most cities can't afford a public works project of that scope. Instead, the ALSA suggests making parks around the undersides of the highways. If you can't tear them down, at least make them pleasant to walk under
Cities with canals may not even know that they have the makings of a pleasant Venetian landscape just waiting to be cleaned up (see above), and there are tens of thousands of miles of out of use train tracks that can be turned into trails. All of these things have benefits beyond aesthetics; they're engines of economic growth. And as the ALSA notes in the video that redesigning old infrastructure is far more effective than starting from scratch. Our cities have the shells of interesting design in them, it just takes a little thinking to make them liveable cities of the future.