The thing that best separates Barcelona from any number of other Mediterranean cities is the architecture of Gaudi. There are a number of sites ( which your guidebook will cover) though IMO the best photographically is Casa Mila (aka La Pedrera) in the centre of the city. In good light the photograohy of and from the roof near sunset is outstanding.

One other thing. The incidence of pickpocketing and other petty theft in Barcelona appears to be high. Look after your things and be aware of the possibility that someone could unzip a backpack without your knowledge.