Pittsburgh take-out near Omni William Penn?
Here are a few ideas within pretty close walking distance, you can probably get more delivered. Keep in mind that the food from the William Penn itself is quite tasty and you certainly wouldn't suffer with room service (except on price).
In general, there's not a whole lot going on downtown for cheap/takeout other than at lunch, and not really the best ethnic options in town, but you shouldn't have too much trouble getting something decent.
Sree's Indian (M-F 11:30-3:00) @ 701 Smithfield St
Mexico City (not sure about weekend hours)
Primanti Brothers @ 11 Cherry Way
Hope you enjoy your visit.