Mountain Montjuic is located at the side of Barcelona near the harbor. From the harbor you can go by an (very old) cable line (‘Transbordador Aeri del Port) to hotel Mirador, the Olympic stadium  of Barcelona. It’s not for the faint hearted(If you are afraid of highs, don’t do it!). From the cabin you have stunning views over Barcelona. On top of the mountain you can take go to restaurants, the old stadium, a castle, or go to the Costa i Llobera Gardens. From the mountain you can go back either by taxi or the cable line to the center or harbor of Barcelona. See link