How can i prevent wrinkles in my dress??

Is there a restroom at the park you can put on your dress when you arrive? Then just wait until then to put it on, and keep it in a garment bag until then? If not, I'd just steam it right before you leave and do your best to not sit on it in ways to make it wrinkle. They also have portable steamers, so that's an option?

If there's a visitor's center and you're going to be there during open hours, it might be worth a try to call the visitor's center, tell them the story, and see if they can help you out by keeping an eye on your dress in the bathroom or hanging it in a closet if you plan on exploring the park before your ceremony.

Best of luck!