Not to derail the thread, where you ask for specific play areas along your route, but we/someone should compile these play areas into some kind of Google Map/App--my very active daughter NEEDS to run around every 40 minutes or so, and we drove to upstate New York and it was very difficult for her, and for us. Didn't think of searching for elementary schools--good idea.
I have no idea how to do this, but even the "Driving With Kids" or "Visiting XYZ with Kids" books and Web sites don't chronicle the little play areas or spaces to run along the routes, or a little off them. Obviously, things change, places close and open--that's why a Web site or app might be best. Heck, a farmers' market can save the day in a pinch, and has, on our way to beaches on the Jersey Shore.
To try to add something concrete to this thread: We really liked this play area, in Glen Burnie, MD:
http://www.aacounty.org/recparks/parks/trails/bwitrailpark.cfm#.VJMsbGB0wW4
Please forgive me if that is nowhere near your route. Happy trails.