Go see Sagrada Familia (Gaudi's surrealistic church) at night when it's lit up. Walk all around it and view it from all angles - no need to go in (anyway, it closes at 7 p.m.). It's full of scaffolding and you'll spend half the day waiting on line to get in, which seems a pity when time is precious.
Eat at a "menjador" which means dining room. That is where the locals eat. It's cheaper and more authentic.
If you have a little more time, walk up to Passieg de Gracia (past Placa Catalunya in the opposite direction from the Ramblas) where two famous Gaudi houses are located - Casa Battlo and Casa Milla. You'll know you're there when you see long lines of people waiting on the sidewalk to get into very wild, lumpy-looking buildings.

