Brian,
There are many little tricks to coax the silver out of a "worked out" park. I hunt the same parks over and over, they do not produce as good as they did 15 years ago. But they still produce. Get to the park early in the morning or when the sun is going down. Turn off the detector since what you are looking for is what the low level sunlight will show you. The park once had more bigger trees than it does now. Those trees were there before the famed pull tab invasion. What you are looking for are slight depressions where a tree once stood 40, 50, 60 years ago or longer. When they removed the tree, or it died and fell over. It was probably cut close to the ground and has rotted away. They may have filled the hole but did not put alot of effort into it. Years later, that would be now, you can still see the depression in the ground where the tree stood. In the evening sunlight, those spots show up. At high noon they are almost invisible. One of the hardest things to hide is a hole. Sounds silly, but true.
After finding these spots, tweak the detector for very little discrimination, if any. Go for all the depth you can get away with a still be stable. And if it has a chance of being a coin, dig! I have coaxed many Barber dimes out of these very places. A small gathering of Mercs, and a few Indians.
Sorry to ramble, sometimes you just need to splain things in detail and splain why.
GOOD LUCK!
Bob