Barcelona 2 days

Get tickets ahead online to La Sagrada Familia for Sunday, your only full day. Are you arriving from the US or Europe on Friday morning? If US, you will deal with jet lag on Friday.

Saturday will be our only full day. Yes, we are coming from the US. But we are good at sleeping on the plane ride over. So, we will be good to go once we drop our bags at our hotel. Ty

I would think about taking a small-group or private tour on Saturday to see the highlights. In 2017, we used this company for 3 tours and highly recommend them:

https://spanish-trails.com/tours/barcelona-city-tours-bcn-small-group-explorer-8-hour/

Here are our host's recommendations for one or two days in Barcelona. The sights requiring advance timed tickets are the Picasso Museum, Sagrada Familia, and the Palace of Catalan Music. You'll probably want to spend your time Friday outside rather than in museums and such. On Sunday you'll want to get to your ship in plenty of time, likely involving a taxi ride to the port which is some distance from the old city.

I enjoyed Barcelona but didn't find the Gaudi buildings as wonderful as many others do. I suggest you look him up in Wikipedia or spend some guidebook time to decide how much of your limited time you want to spend admiring his buildings. On the other hand, if you're a fan, you could give Gaudi a whole day.

The Picasso Museum focuses on his early years in Spain, with less attention to Cubism and his later works, except for those late in his life. I found it very interesting, but some might disagree.

Other sites requiring pre-purchased tickets are Parc Guell (but reportedly much is under construction now, though I haven't been there recently), Casa Mila/La Pedrera and Casa Batllo.

Although the Picasso Museum is interesting to me, it can be mind-bogglingly crowded and I do not recommend it for anyone who isn't a big fan, especially those with little time in the city. There are many more-accessible museums in the city. MNAC is great (and large); for modern-art fans the Miro Museum is more pleasant to visit than the Picasso because it's much less crowded. And there are many more.

Things you can do without pre-purchasing tickets: wander the Barri Gotic (walking tours available), visit the Sant Pau modernista site, see the Cathedral.

I too arrived in Barcelona on a Friday and left on a cruise on Sunday. We spent some time reading up on the sights to see what appealed to us. Rick Steve's book on Mediterranean ports was very helpful. I think it would be easy to spend a week in Barcelona - with a brief visit you'll get a sample.

On Friday we dropped our bags, ate lunch, and had a 3 pm appointment at La Sagrada Familia. Then we explored a bit until it was late enough to eat dinner (it's much later than we were used to). Saturday was rainy, so we went to the Maritime Museum (I am daft about boats, this won't be on anyone else's must-see list). They have a lovely cafe. On the walk back I whipped out my list of wine stores and visited one to purchase the two bottles you're allowed to bring on a Carnival ship (one per adult). We had a 6pm tour of Park Guell booked, and the rain had mostly stopped by then. As others have mentioned, the major Gaudi sights require pre-booking, so scheduling them can be tricky.

Was just at Parc Guell, there is construction but not much. I felt it was still worth the admission, but with such limited amount of time I'd say not to go so far afield and stick to something that'll eat up less time. Unless you really want to see that spot in particular, of course!
I really liked the RS El Born walk and Barri Gotic walk and I'd agree that Sagrada Familia is essential. It blew me away.
Those walks might be a nice way to orient yourselves and fight the jet lag!

Please don't miss Sagrada Familia. It was so incredible I wanted to stay and stay. Do book tickets FAR in advance. We really enjoyed Parc Guell but getting there and touring it can eat up quite a bit of time. We enjoyed Casa Mila more than Casa Batllo. The Recinte Modernista (Hospital de st Pau) site was beautiful and free on Sunday. You can walk there from Sagrada Familia. We stayed at the Hotel Continental Palacete and almost everyone was waiting to depart on a cruise. Great location and included 24 hour buffet.