Voy,
Bascially you are aiming the antenna at the park's wifi source, collecting and amplifying the signal - correct?
What antenna are you using and what type of output cable does it have - coax?
Most routers usually have coax input from a cable modem. However, in your case, the router you use will need to be configured as an access point where by it receives an IP address from the park's router.
More to it than that and there are some really good poster's here that I am sure can offer more details and explanation.
However, a bit more information is needed: E.g., will all of your devices being wired? Or will some of them still be wireless? The end configuration can get a bit complicated.....