Go to the Specialities-Market infront of Viennas City Hall. There is a vast restaurant area during the Vienna Festival Weeks !
If you prefer genuine Viennese fare, try "Rathauskeller" a medium-priced reestaurant in the basement of Viennas CityHall
Don't miss Figlmüller restaurant. They have a couple of branches in the city center and their schnitzel is heavenly :)
A fun place for lunch or afternoon tea is Demel (www.demel.at; Kohlmarkt, 14). It is right in the middle of everything, with great food and service. Try to get a window seat looking out onto the Kohlmarkt.
Deffinitely agree with the previous answer, go to Demel and try the postries with Melanche coffee: "Eat in a pastry shop" - Vienna Things to Do Tip by mirchica