I accidentally clicked on "Canadian Trip Planning..." instead of "Transportation," and happened to see your thread. I live in Pittsburgh, and there is a lot to see and do here for a family. I don't know your age ranges, but here are some suggestions:
1) Within about 5 minutes or so of Point Park (the place where the rivers all come together), you can see:
a) The Carnegie Science Center (lots of interactive fun),
b) The Children's Museum,
c) The National Aviary,
d) Funiculars ("the incline") that run up and down Mt. Washington,
e) The Andy Warhol museum (best for older kids - there are arts programs on the weekend as well), and
f) a Pittsburgh Pirates ball game.
2) Elsewhere in the city, we have:
a) The Carnegie Museum of Natural History and the Carnegie Art Museum in the Oakland neighborhood, and
b) The Pittsburgh Zoo and Aquarium in the Highland Park neighborhood.
3) There are also numerous outdoor activities such as biking along the riverfront.
All of these places are google-able, if you want more info. I also don't know when you are traveling, but if it is around Sept. 24-25, I would recommend NOT stopping in Pittsburgh, as the G-20 summit is in town then, and things will be crazy busy.