Been to Barcelona recently for long weekend but still enjoyed short stay.
HINT Buy a T10 ticket at any Metro station from an automated machine for under 10E. It can be used by a couple for 2 X 5 journeys on the bus or metro/funicular. We were warned about Pickpockets etc but with sensible precautions travel on the metro didn't appear in anyway threatening. We would use it again without hesitation but with care.
We explored Mont Juic. Use the Funicular to get up.
The 12E entrance to MNAC (Art Gallery) was worth every cent: fantastic frescos and Modernista furniture.
A bus up to the castle gives fine views over the port and the city.
We had some spare time and spare T10 journeys, so we used the metro to revisit
Sagrada Familia and see how much more has been added to the work in progress.
Finally a metro back to the top of Las Ramblas. Walk down to Columbus statue. If you are with a sensible cruise line, one third of the way down on the left is a Carrefour which sells very reasonably priced alcoholic beverages.