1.) La Rambla
You probably should stroll down La Rambla at least once if it’s your first time to the city. Once is enough. La Rambla is a thorn in many a local’s side, and all the junky souvenir shops selling Mexican hats.
2.) Flamenco and Dinner
We’re not in Andalusia, which means were not in flamenco country. Avoid any “flamenco” shows in the center. A great flamenco festival is in Barcelona now through March called De Cajón.
I don’t know if sangria qualifies a tourist trap per se, but it isn’t anything a local would drink. However, “cava sangria” is popular in Barcelona and I would recommend trying a glass or a pitcher of it. Unlike traditional sangria, cava sangria is made with white cava or rose cava (sparkling wine), and usually has less fruit in it.
4.) Sagrada Familia
With more than 3 million visitors entering its gates every year, Antoni Gaudi's Sagrada Familia is the most visited tourist attraction in Barcelona. Unfortunately, the admission fees and queues to get in are at least equally impressive as the yet-unfinished cathedral itself. As an alternative you can visit, Torre Agbar; located between Av. Diagonal and C/Badajoz and looking rather unspectacular during the daytime, this 38-storey tower is a real eye-catcher after dark, thanks to its colourful nocturnal illusion.
5.) Bus Turístic
If you're looking for a more individual and flexible way of sightseeing and wouldn't mind going without a tour guide, grab a guidebook and go exploring via the city buses that take you through Barcelona on similar routes to the tour buses, but for a lot less money. Advisable lines include bus number V15 from Barceloneta to Vall d'Hebron as it goes along Via Laietana, Passeig de Gràcia and Diagonal, or number 13, which covers almost the entire area around Montjuïc and Plaça d'Espanya.