What ever you feel like wearing that evening would be fine. Only attire that might not be appropriate would be a bathing suit --I understand tank-tops are not welcome either.
You'll see all sorts of dress there--from what we always wear (theme park casual--nice walking shorts, tennis shoes, and a Disney t-Shirt), all the way thru jackets for a few men and a dress for the ladies.
So wear whatever you feel most comfortable in and don't worry--you won't be dressed "inapropriately" and no one is even going to notice--or care.