If your flight is from Barcelona and you want to spend the day there, here is the answer. Leave your luggage here behind Plaça Catalunya behind Cort Ingles), while you wander round the city. (Also useful to know is that there is left luggage in Sants train station.) They speak English, are very friendly and helpful and it's an extremely good location. Also cheap and no closing time during the day, so you can pick it up when you want.