This bar is located on the terrace of the Gran Hotel La Florida in Tibidabo (a deluxe hotel where John Malkovitch stays). Tibidabo is the highest mountain range in Barcelona (520 meters high) and is situated in the Collserola natural park. This explains the breathtaking views (see picture) from the bar. There are tables where you can have a light lunch or a drink, a sofa, an outside counter with stools.
The best moment to go is during spring and autumn during a sunny day when it's not too hot.
It's an elegant place, quiet, pricey (also to have a light lunch) but worth the view.
It's easily accessible by public transport so just take a cab.