I never had an issue with lines for the pandas but I visited a bit later in the day so that lines up with what
@ZooGirlSD. I even saw one walking around!
First thing's first give yourself two days here, you'll probably need it. It's one of those zoos that you could get done in one day if need be but you wouldn't be able to take your time with it and probably wouldn't have time to revisit enclosures for any species you missed. The only other zoos I've had this problem with are Bronx and Chester as the latter rapidly develops and expands.
If you want to see the one and only Tongan Scrubfowl in captivity I'd go to her enclosure as soon as the zoo opens. It's on the far side of the zoo from the entrance which is annoying as you have to skip through a lot (including Aviary Trail which is hard) but her enclosure has a slope that she hides in pretty much from the moment visitors show up to when the zoo closes, except for when she gets fed but I don't know when that is. This also brings up a very important note if you're a bird lover: don't miss the hidden row of aviaries! The aviaries are right next to the Arctic exhibit complex if you take a right out of it but then take another right down the hill into what looks like a keeper only area but isn't. If you turn left you'll head past the Giant Eland.
I found that Owen's Aviary is best towards closing time, that's when I saw most of the odd birds in there. It's hard to find everything in one go, though, so that's why revisits are crucial.
Don't skip the children's zoo if it's still open, there are some real oddball animals there. And definitely don't miss the little confiscated reptile/arachnid building that's in the middle. I don't know if the hummingbird aviary if still open but if you that's worth visiting, too.
~Thylo