You can take I-37 south to Hwy-358, which then turns into Park Road 22. After crossing the JFK Causeway, stay on 22 all the way to the entrance of PINS.
Here is a map of the directions to Mustang Island SP (also in the area), which should help.
http://www.tpwd.state.tx.us/park/mus...es/mustloc.pdf
I learned the hard way about coyotes there. Awoke one morning to the sound of our cooler being dragged away! They are there, so take heed.
