I have been visiting this park for years with my family, and love the features - one of which is prominent with this location. It reminds me of something I did with my Grandfather, but unfortunately we usually did it indoors though I know if we had something like this he would have taken me there to play.
You are looking for a magnetic keyholder. There are two possible locations in close proximity to each other, so you may want to check the hint.
Enjoy!