Barcelona

You make no mention of your interests. Use the Rick Steves itinerary suggestions in the link below and based on what interests you, you can create your own itinerary.

https://www.ricksteves.com/europe/spain/barcelona-itinerary

I emphasize two things of the dozens I did in Barcelona.

First, Go on the Barri Gotic walk that Rick has in his book. The Barri Gotic is, of course, Barcelona's Gothic Quarter. Rick points out 20 or so sites in the quarter and tells you about each of them. Two fascinating sites included an former royal or government building that in 1492 or 1493 received Christopher Columbus on his return trip from what would be become America. And the Jewish quarter that once housed tens of thousands of people in an area comprising a handful of city blocks.

Eateries: any restaurant that sells jamon Iberico (Spanish ham) is probably good. All I ate in Barcelona for four days was jamon Iberico.

I also enjoyed a side trip from Barcelona to see the Salvador Dali's studio and museum in Figueres, which is about 90 minutes north by train. The town itself was worth the visit, plus the museum is fabulous. I'm told his home in nearby Port Lligat is worth seeing. It's possible to take a bus from Figueres to Port Lligat, but it isn't easy. Unfortunately, I didn't see Dali's home.

These Barcelona sights have very long ticket lines you cannot afford to stand in, so decide ahead of time whether you want to see any of them. If you do, buy tickets online in advance. It's probably not safe to wait until just before you arrive in Barcelona for some of the tickets, but I don't think any of these places sell out far ahead of time. The difficulty is that the tickets are timed, and the early slots tend to be the most popular. If you wait too long and all the early-morning tickets are sold out at these sights, you suddenly don't have a full day available for your sightseeing.

La Sagrada Familia
Parc Guell
Casa Mila/La Pedrera
Casa Batllo
Picasso Museum

In addition, the English-language tours at the Palau de la Musica Catalana can sell out.

Just returned.

Loved Montserrat but that would be half day minimum.

Also loved La Familia Sagrada.