Buy an ISIC Card.
The short answer is: in very few places. The ISIC card is not widely recognised in Spain. I wouldn't recommend buying it especially for Spain. If you already own one, then you will find where you can use the card below. If you don't have one yet, consider buying a different Barcelona Discount: Barcelona Discount Card Comparison
The ISIC Card can be used in Barcelona at Partybus, Fresc Co, Hard Rock Caf�, Restaurante Salta, Travel Bar, La Casa del Libro, Rosa Negra, Fitness First, Elipsos Trainhotel, Avis, Racc, Viajeoteca, Viajes Azul, Viajes Barcelo, Viajes Ecuador, Viajes Zeppelin.
Buy an ISIC Card.

