The zoo is as actually opening two hours early, at 8am, for members, tomorrow through Sunday, to allow them to preview Hippo Cove ahead of the crowds. That said, I'm not sure whether this will just mean that the attendance will be high, even in the morning, or if it will be heavy early and taper off later on, with members getting in to see the hippos and then getting out... I would, thus, probably agree with Moebelle. There's going to be an early rush for the hippos either way, both for the members at 8 and for the general public at 10, so I would say any time after noon would be your best bet, visiting the less frequented areas of the park first (Wildlife Canyon, Wolf Woods, Dragons, etc) and saving the generally more crowded areas (Africa, Elephant Reserve, Gorilla World, etc) for later on in the day.