A micro-sized camouflaged cache hidden in a way that - hopefully - avoids detection by local kids and/or muggles.
While you're here consider the name of this park. It's named after bakelite, the material which is an early form of plastic. It was used for all types of household items and was the basis of a sprawling manufacturing industry in the surrounding neighborhoods. If you're interested in learning more, read up on its history on Wikipedia http://en.wikipedia.org/wiki/Bakelite .