I know that feeling almost exactly. ABout a block from my house was 1970's school park which i hit heavely. Over the four years that i've hunted it i got some strang finds that include:
A 1920's watch fob
A semi-melted lightened rod
An old gold Earing
An old crushed bell
an a slew of farm junk
so, i was expecting a small park with modern junk, but ended up with some of my oldest finds