I was in Barcelona earlier this year for 3 days and there is so much to see we didn't get to do it all. We wandered round all the little shops on and around La Ramblas (the main street), visited Casa Mila and Casa Batlo (both buildings built by Gaudi), had a lovely afternoon at Park Guell, which again has lots of Gaudi stuff including some fantastic wavy benches and to complete the Gaudi stuff we went to see La Sagrada Familia. One evening we visited the musical fountains which were great, before hitting the bars and restaurants! Sure you'll have a great time in Barcelona.