Interesting park i used to metal detect in as it was built in the Mid 1950's so some silver coins have been found here ( Google when American stopped putting silver in its coins )
When ever i came to the park i always found this one spot i particular interesting. there is always some interesting form of insect near by. You will have to expand your idea of where you think a cache might be hidden to find this one, as it is very well camouflaged.
I will add a hint to this if it proves too difficult to find.
This is very suitable for small children, i would encourage you bring them along, but bring a small/slim item for the exchange.