If you have only half day to visit Barcelona, I recommend the following tour.
Itinerary
Gothic Quarter, the Ramblas, Casa Batlló – Casa Milà (outside), Sagrada Familia (inside).
Duration
4h
Transport
By foot and public transportation or taxi.
If you have a whole day to visit Barcelona, I recommend the following tour.
Itinerary
Montjuïc Hill, Gothic Quarter, the Ramblas, Casa Batlló – Casa Milà (outside), Sagrada Familia and Park Güell (inside).
Duration
8h
Transport
Chauffeured car or minivan.