I just returned from a trip with a friend and we had 4 nights in Barcelona. We rented a vrbo - but be careful that they have a license... our first airbnb was canceled I think because there was a crackdown. We rented from Jose, he has a lot of apartments, and he was great! He met us at arrival and departure, no problems at all. Language no issue, he spoke english, and really we had no problems in Barcelona at all getting by with very limited high school Spanish.
This is the one we stayed in, but looks taken - email him for others. /www.vrbo.com/241486 We just got back 2 days ago! :) I would look for a place with air conditioning because it was HOT. He was SUPER nice and helpful!
As for the beach, we had planned to go but we walked down there to check it out the night before and it was not our scene (I am 45, she is an older-soul 32)... in your 20s maybe you will like it. It was definitely a Miami South Beach vibe, very different to the rest of Barcelona, so we skipped it. But the young people looked to be having fun! :)
Book tickets in advance or you will not get into the major sites. We booked Sagrada Familia, Parc Guell, Picasso museum (we booked these day of to avoid the line) - if you want to visit the insides of the Gaudi houses you should book those too. We saw a couple Flamenco shows (one at Tablao Flamenco Cordobes and one at the Palau de la Musica). We ate a lot of awesome food too!
I have a lot of details in my travel blog if you want to check it out... look for the ones "Physics in Europe 2017" (we are both science teachers...). Still have to go back and add photos to my other trips... I had to change websites. I think I named all of the restaurants in there.
https://onatangentweb.wordpress.com/
Have a good trip!
Kim