Barcelona offers not only much for the culturally interested, there's also a lot of nightlife.
Start the party life leisurely. After lunch shopping along the main shopping streets, then at about 21:00 to dinner in one of the many restaurants, then in a bar, after this from 1:00 in a club or disco.
In Spain, people usually eat very late, but some restaurants open the kitchen earlier. The bars are often closed at 3.00, before 1.00 you do not need to go into a disco if you don't want to be alone on the dance floor. You can then dance until the early morning hours, then you can eat breakfast in a cafe.