For a bank holiday weekend we booked this trip with a tour operator, flights & 4 nights hotel for £250pp. The city is full of beautiful architecture, nightlife and great food, and right on the water too! We went on a Thursday evening and stayed by the harbor, not too far from Las Ramblas the main square.
You must visit the famous Sagrada Familia cathedral started by Gaudi and still under construction. Other Gaudi buildings were interesting but nothing is a fascinating as this church.
Another must see is the Picasso museum. The queue will most likely be long but it moves fast and is worth it to go inside and see some of Picasso's early work.
Parc Guell is a beautiful place to walk around and enjoy the Gaudi architecture. There are nice city views from the park.
Of course a trip is not complete without stepping into a tapas bar for a quick bite and some vino.
There is so much to do and see there you will probably want to go back again.