First you should pack a backpack or purse. My mom usually carries one for my whole family. You will need to hold your park passes if each person does not have a lanyard.
Personally, I like to wear a t-shirt, athletic shorts, and sneakers to a day at a theme park.
Sneakers are a necessity if you plan to walk around to all the rides!
Another option is a little more formal but still comfortable. Jeans shorts and a nicer shirt is also a good option, as long as it is comfortable for you.
I do not own this shirt/sweatshirt but I honestly love it! plmonograms.com
Pack a backpack to hold your stuff.