If I had to choose one singular and authentic bar in Barcelona, I'll say "Mendizábal".
This is not a fancy bar, and that's what I love of Mendizábal. But it doesn't mean it isn't a cute one with a beautiful decoration and good prices.
They have a two very differentiated spaces: one privileged terrace in the city center, and the interior salon. Them both are awesome, and it's perfect because you can decide which one fits better with the situation.
They do tapas, sandwich, juices, burgers and veggi burgers.
In my opinion, they have the best vegetarian sandwich offer in town.
But the most important, you can leave the bar without eating their "patatas bravas"
City-beach with very different atmospheres, depending on the season. In the summer its crowded and full of people sun-bathing, partying, rollerblading, swimming etc. But in the winter it can be a wonderful quiet place.
The "1929. The Barcelona Pavilion" is one of the places where I always take my friends when they come to visit me to Barcelona. It was designed by Ludwig Mies van der Rohe as the German National Pavilion for the 1929 Barcelona International Exhibition. The day I made the photo was raining heavily, even then I found it great.