Parks are great places to explore, exercise and socialize. A nomination can be made for a park even if a park does not have a physical sign but is clearly a park. If there is an obvious entrance or focal point, that would be a good location to place the pin, but equally the middle of the park may be suitable. The supporting photo and text should provide evidence that the location is intended to be used as a park or recreational area.
The image above is a good example of a park that lacks a sign but placemarks a park.