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"
Sunny days I like to go to the Palau Robert Garden. Is an small garden placed in the middle of the city. The palace works today as an exhibition space, so after reading the newspaper under the sunshine, you can visit an exhibition. I love to go there to feel myself surrounded of abundant vegetation and the songs of the birds. It makes me be in a good mood.