Barcelona is full of such great art, obviously the Sagrada Família is a must-see, but my personal favorite place to spend a half (or even full) day is the Parc de Guell.
You can really see the size of the cathedral in the skyline from the park too, it’s located on the top of a hill so there’s a vista of the whole city and ocean, it’s amazing. Speaking of, get down to the Playa.
You can play volleyball, grab a bike from a little bike renter machine, or just walk along the beach there and grab drinks & tapas from one of the beachfront cafes. There are also nightclubs by the beach there.. (I think we went to Opium Mar) which stay alive until 6 or 7 in the morning (convenient, trains start running again then!). The best neighborhoods to wander are the Barri Gotic and Born. The walking around those areas during the day is really good, lots of interesting shops, foods, galleries- you can’t go wrong. There is a plaza off La Ramble (the big street starting at Plaza Catalunya), Placa Reial, that is pretty touristy but a good starting point when heading out in the Gotic neighborhood in the evening.
Drink sangria by the pitcher! At tapas, try the jamón and the patatas bravas. For a feast, get (seafood) Paella. After dinner, my favorite drink is called a vermut casero. If you like art, there is a night each month when galleries all open their doors and serve tapas & drinks- you’d have to ask around to see when that’s going on, we lucked out & went, it was really cool (we went in the Born district). Oh! And last but not least, there is a really giant Picasso art museum in Barca- the art is really what the city is about. And the food/wine. Decent bottles of wine start at ONE EURO so don’t waste time- try as many as you can! I am generally a red fan, but there is a unique Vinho Verde which I thought was quite good- try it! Be careful for pickpockets.