What a great set of museums! You have art, the natural history and the library all in the same building on a sprawling campus by Pittsburgh standards!!
It is cheap to get in (it used to be by "donations", but I'm not sure if it still is... you can also buy into a membership program) and it is a really awesome way to spend an afternoon!
There is a cafe in the art museum (and if I remember correctly, a cafeteria in the natural history) that serves sandwiches and the like.. there is also a nice sized gift shop.
Metered parking outside or a lot that you can enter near the entrance to the art museum is a great option. The building itself is impressive looking.
Enjoy!