Plan on seeing lots of animals and getting a workout when you come to the Pittsburgh Zoo. It's a huge, beautiful park with lots of hills and exhibits.
There's a walking path you can take to avoid the long elevator lines at peak hours, but it's gravel so if you're taking a stroller, make sure it's a jogging stroller.
We have a routine when we go-animals first, playgrounds/lunch last. The Islands exhibit is so cool in the spring/summer months because you have access to the huge sand area, and the swinging monkeys are a hit.
We always skip the indoor monkey exhibit because it's super stuffy and smelly in there.
The aquarium is our favorite though. My kids stand in amazement every time, and they laugh so hard as the penguins do their thing, and the lower floor is the coolest because it has the largest fish.
The playgrounds are awesome. There is one near the sea lion exhibit with all slides and the huge climbing nets, and there's a newer playground near the kangaroo exhibit. My kids could spend hours at these playgrounds alone.
Come here right when the park opens so you can get in and out before the crowds are crazy.