We spent several days in Barcelona last summer & saw many sights. By far the most impressive was La Sagrada Familia. It is unlike any other cathedral in the world. Beautiful & innovatively designed, particularly the interior. That is the one thing I would classify as a must do in Barcelona.
Park Guell & the other Gaudi sights are very nice & worthwhile.
Gran Via de les Corts Catalanes is a good, centrally located street for your hotel. Cotton House is a very nice luxury hotel there & we liked it a lot. There are other hotels there at different price points.
Las Ramblas is just a tourist strip, with not much to see. Only saw one human statue when we were there- there has been some kind of crackdown on them, so they aren't as plentiful as they once were, unfortunately. Imo Las Ramblas is only worthwhile if you're going that direction anyway, or if you are nearby & restless & just want to go for a stroll. It has energy because of all of the people, and many cheap & quick places to eat.
Barcelona has the reputation of being the pickpocket/petty theft capital of Europe, so be extra careful with your wallet & phone. Don't rest them on cafe tables, leave them in backpockets, or carry easy to snatch purses, etc. Don't talk to strangers who try to engage you. We were very careful & had no troubles, but I do recommend being careful in all of Europe, especially Barcelona.