Barcelona is where more Mediterranean itineraries start and end than anywhere else, which means that for most people it is the day before or the day after the cruise rather than a port day.
Treat it as a city break, not a call. Coming in a day early is the single best decision available on a Mediterranean itinerary. It absorbs a delayed flight, and it turns Barcelona from a taxi ride into two of the better days of the vacation.
Book the Sagrada Família before you book anything else. Timed entry, sells out weeks ahead in season, and it is not remotely the same building from the outside. Park Güell is also timed now and also sells out.
The cruise terminal is not walking distance. The Adossat quay is about two miles out along the mole. The blue Portbus shuttle drops at the Columbus monument at the bottom of the Ramblas; taxis are metered and cheap.
Pickpockets are a genuine, organized problem on the Ramblas, in the metro and around Sagrada Família, not dangerous, but professional. A zipped bag in front of you handles it.
Eat late. Lunch is at two and dinner starts at nine. A restaurant serving dinner at six thirty is a restaurant for people off ships.