The Old Port at night is better than ever during La Mercè. There's a cava and wine tasting (XXIX Mostra de Vins i Caves de Catalunya), complete with generous tapas of the traditional kind. (It's no use reminding yourself that the wine is for sampling not inebriating.) You can see the Barceloneta fireworks competition from the pier. And, schools of tiny sparkling fish put on a show below your dangling feet as they try to escape their determined predators. In other words, there's a whole host of entertainments that I highly recommend.
The cava and wine tasting continues until Sunday. Plan to go early in the evening as it closes down at around 10:00pm nightly, just before things get really raucous; that means that you have to purchase the tickets that you trade in for glasses of wine and tapas before 9:45pm. Once equipped with wine and food, you can spread yourself out on the pier, with blanket or sans, and enjoy to your heart's content. You'll know it's time to go when someone pulls out the bagpipes.