Melbourne does this well with a system of bike and walking tracks along the city's river, the Yarra, and its tributary creeks, Merri Creek north of the city, and Gardiner Creek, in the city's south east. Community groups and local government combine to maintain the trails and vegetate and re-vegetate the areas as needed. Some parts of the tracks are bushland, other parts present urban views.
The High Line in New York City is a park that is still being developed on the site of a derelict railway line. It is thirty feet above street level and over a mile long. It's not a park in the traditional sense, i.e. a natural space within a city. It is, rather,an inspiring example of combining the natural and the built environments.
Parts of the line are secluded natural spaces, other parts run between buildings. The designer, Piet Oudolf, specializes in wild gardens, and has planted natural looking drifts of perennials, mostly indigenous species. These are supposed to echo the weeds which grew alongside the line when it was a functioning railway line.

Walking the High Line must be a wonderful way to see Manhattan... sigh ...
... but in the meantime, I'll just wonder down to Gardiners Creek, and see if I can see any of the rakali (native water rats) that live there.


(photos from Wikipedia)