Barcelona has always been a great city after dark. The Tourist Information offices are a great source of info, as is What’s on Barcelona-Metropolitan and We Barcelona and the city's English language BCN Guide. See here for tickets.
Pick up same day half price tickets at the Palau de la Virreina.
Unless you speak Spanish or Catalan and can take in the city's very lively theatre scene or a film (there are quite a few Cinemas) that regularly show movies in their original language, your night out will probably involve music. Also check the list of Festivals and Events: it's a rare month when there isn't something to tempting on offer.
Image by Ryan Mcdonald