If you only have time to do one thing in Barcelona, you should see the sound and light show at the Font Magica of Montjuic.
It runs in the evenings from Thursday to Sunday in the summer and Friday to Saturday in the winter.
It is wonderful and it is free.
La Font Magica de Montjuic, Barcelona, Spain
Same Skies Above 
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Super excited. Barcelona has been on the top of my travel to-do list for some time. Now I know what I’ll have to do when I get there. Thanks for sharing.