Wed night and Sun night you have a Tipico band in D'Franks rest/dance hall. Usually very good. Mon, Tues, Thurs only good for a quiet drink in a bar. Fri - can get a bit more lively and Sat/Sun is hopping.
Weekdays you can find action in the Barrios, very rough, cheap and ready. Romanita's a Colmado with no sign. Head away from the lagoon and D'Franks and keep going until you find yourself in Barrio Buenos Aires - you'll know you're there, and just before there is Colmado Espanol.
Bahia Blanca is easily the best place to stay in town. I like the room downstairs, is much cheaper (500 pesos) metres from the sea and more private than the rest (a huge balcony to yourself). Its cheap cause they hang the washing out there. The room is number T1.