For something on the simple side, City Extra at Circular Quay may be OK. Their upstairs dining area is hardly every busy. 'American Diner' style of food in generous portions.
For something nicer, look at Morrison's Oyster Bar and Gril in George Street. Less than 5 minutes from Circular Quay. Upmarketish pub fare. Several areas and private rooms. FRIDAY afternoon and evening may be BUSY.