Larger parks, of course, usually have more options. When the lights go off, even wide open fields in sight of homes or streets are possibilities. Bushes, benches, fences or seats may screen good spots. This is one of my favorites. If you don't mind sleeping with those who will never wake, you'll sleep undisturbed.
Floodplains usually host parkland or ragged wildland and feature good camp spots.
Rail corridors into and through cities are usually lined with vacant land that can host sly campers. The margins of big switching yards offer the best chance to avoid being shocked awake by a streaking express. They're everywhere, though few are inspiring places to enjoy a relaxed night of sly-camping. Keep your eyes and mind open. To be a successful sly-camper, you should think like a spy.