So much to do!
Sagrada Familia is amazing - if you can't get tickets this late, it's still worth a look from the outsides. Guell Park is lovely. Wander by the beach and have lunch or drinks by the sea. Head up by funicular to Monjuic - I particularly liked the Fundacio Joan Miro, up there, and the olympic swimming pool was interesting - you can have a beer while looking past the pool and diving boards out over the city.
The cable cars are fun, and we also liked the Time Out Market. There's a square off Las Ramblas called Placa Reial which has lots of nice restaurants and a buzzy atmosphere in the evenings. If you are into football, Camp Nou is worth a visit (I'm not, but my DH enjoyed it).
Take a trip out of town to see the cava wineries, or head to Monserrat. Or get out on the water on a sunset sailing trip.