We spent a few days in Barcelona in 2019, and I strongly recommend visiting (inside, not just outside) La Sagrada Familia. It has a striking exterior, but the interior is even more beautiful, due to the effect of the multicolored stained glass windows when the sunlight streams through. If you're visiting on a Sunday and are a Christian, you might enjoy celebrating Mass there. We did so, and it was a very special experience.
Parc Guell is beautiful, and Casa Mila and Casa Battlo are also worth visiting. Montjuic Castle is very nice, too, especially since it offers a panoramic view of the city.
Las Ramblas is overrated, imo. The famed human statues were few and far between when we were there, having been mostly replaced by cheap trinket stalls. It's fine if you need to walk through it on your way somewhere else (it's interesting to see the Christopher Columbus statue at the base of Las Ramblas, for example), but it isn't a significant destination of its own, except to be able to say you've been there.
Be very careful about pickpockets in Barcelona, and ignore beggars. We were careful, and never lost anything. But many less cautious tourists get ripped off, so be careful.