You can't leave Galicia without sampling its superb meat first.
One of the best places to eat a T-bone steak is El Asesino (Praza da Universidade). The veal T-bone with French fries is exceptional (30€ for two people). For starters, try the house Empanada gallega (5€) and the "vieiras" (6€ per scallop). Dinner will run you around 30€ per person.
If El Asesino is closed (it closed in 2013 due to some problems with the owners of the building, but they will probably re-open their doors soon), another option is Asador Gonzaba (Rua Nova de Abaixo 17). It is a bit far away from the historic center (though at walking distance), but it is quite famous amongst locals and the T-bone steak is gorgeous. Dinner will also run you around 30€ per person.