Paco Meralgo for tapas. Teoric for small creative plates. Reserve for both places. Or, for Paco Meralgo, show up when they open the doors and sit at the bar, which is fun. Santa Gula has great food and a fun vibe, but it is small and be sure to reserve. Rebelot--a small bar/cafe in Barceloneta. We loved the funky atmosphere and great small plates. Be sure and try vermut somewhere. Morro Fi or Casa Mariol are good choices.
I'll piggyback on here. We will have, returning to the U.S., seven hours between flights and were thinking of going into the city for lunch. It will be a Monday, which limits the options, in September. Based on previous posts here, I'm focusing on Senyor Parellada. It looks good for Catalan cuisine; we could take the airport bus to Plaça de Catalunya and have an interesting enough walk. Is there any reason to discourage this plan?
I don't know about the route or timing, although it seems like 7 hours should be plenty. We've had two wonderful lunches at Senyor Parellada and recommend it highly. It looks very formal but has great service and delicious Catalan food.