Pittsburgh zoo is pretty cool.. it's downtown but ontop of a plateua (sp?).. you have to go up a HUGE escalator to get into it.
It has a meerkat exhibit where you or your kids can climb into it and poke your head up in this globe so you are amongst them..
Also a new aquarium.. built maybe 6 years ago..
A HUGE elephant enclosure..
My kids LOVED going there when we lived in PA.
For a family you might want to consider getting a zoo memebership. For me it was actually less than a regular admittance for the entire family (of six) but it let us get into MOST of the zoo's nationally for free for a year, and I think it gives you a good discount at the Baltimore Aquarium.
We used to split our Zoo time between Erie PA and Pitt..