Some good things mentioned. If you tell us 'locals' which specific day, we can be more helpful. Feel free to email me personally if you're worried about saying this in public.

The Fallingwater trip will take up most of your day and is a nice option assuming the weather complies.

And the local shopping area is 'Robinson Town Center' not Century III. RTC is 10 mins from airport.

I have season tickets to Pirates/PNC Park. If I know better which day you're around, maybe I can help. But most likely you would need a day game. What are the times of your layover?