We have been twice....once on New Years Day, and once at the end of August.
For our August trip, we brought water shoes and were glad we did. The sand (and some of the pathways) can get very hot if you are blessed with a steamy, sunny day like we had.
A PP mentioned about bringing a hairdryer, but there is one (or maybe two?) in the ladies shower room. So that's not 100% necessary to bring along. I have long, thick hair and managed just fine with the hairdryer at DC. And surprisingly, no one else was waiting to use it as they dressed and left

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I would definitely bring a waterproof camera (even if its one of the disposable kind) or plan on purchasing one when you arrive at the park (I think they are about $14).
Bring hats. For a summer visit, you will want to avoid sunstroke.
Other than that, you won't need a thing besides your confirmation of purchase and some cash or a credit card (if you wish to purchase the photo package or some souvenirs in the gift shop). Arrive early, plan to spend the WHOLE day, and enjoy an amazing experience!