Absinthe and Barcelona!
Barcelona through its connection to the arts and France does have a rich and interesting absinthe culture. Unfortunately it also has a great deal of imitation products made for tourists.
Instead of buying a flask of neon mouthwash, I recommend searching for the real thing. It is a fun adventure in and of itself.
Bar Marsella is a famous 200 year old bar located in the slums of Raval. The place caters to locals and eccentric tourists and serves a home brew absinthe made in the basement. The absinthe itself is ok but the experience is well worth the slightly dangerous walk.
Las Cueva Del Sorte
Calle Giguas number 2
Is a fantastic cocktail bar that looks like the inside of a melted candle. It is again a locals place.
The bar is amazing and they sell a brand of real absinthe called Obsello. If you ask the bartenders for it they usually prepare it in a elaborate flaming thing that includes inhaling the vapors of the drink (ewww). A little unconventional but a great experience. Plus the more absinthe you drink the more interesting the fairytale architecture becomes.