I have been to Barcelona twice now - both within the space of 7 months of eachother, and I definitely haven't been for the last time. Not only is it perfect weather wise, but it has everything you could want, all in one place. We stayed in Barceloneta, which gave us easy access to the beach, the city, and also nightlife...all walkable (except when in heels).
We spent our days exploring Barcelona, broken up with afternoon naps on the beach, and it can't be disputed that we ate and drank like kings. Terraza La Isabela was so perfect and gave me another memorable rooftop experience when I saw a shooting star as the sun was setting over the city. They also provide you with blankets, and for someone who is as "nesh" as me, this was crucial.
We used Air bnb to organise our stay in the apartment - which was perfect and our host was lovely. The whole apartment was clean and modern, and we even had our own private terrace where we would sit and sip red wine before getting ready for our evening out. In terms of bars, Ocana has to be one of my favourites. This is located in Placa Reial, and has its own underground bar complete with fairy lights, cocktails and its own photo booth. We went from an underground bar (hiding from the torrential rain - but we will overlook that detail) to the Eclipse skybar at the top of the W Hotel...opposite ends of the spectrum! The atmosphere in the sky bar was great, and gave us a completely different ocean view of Barcelona.
If I could recommend one thing to do whilst in Barcelona, it would be to get yourself to Surf House and order some nanchos. You will not regret it.