@missingdata It‘s been six years since I’ve been (was supposed to go in 2020 but that didn't work out…) so things may have changed a fair bit. That said I have still have friends there so will ask them for a few pointers on top of what I know and see if they can help out =)
One great thing about BCN is you can walk around and find amazing things in each district. They all have their own feel to them, if it's history or cool things to see/do/eat/shop in. Some of my favourite places:
Gracia - amazing place for shopping and eating in. Bit expensive and generally regarded as a hipster place. Lots of great restaurants but you will pay a bit more. If you want a chilled evening with less partying people it's a great place to hang out in. There's a lot of streeside restaurants so you can sit there and watch people while chilling.
Arc de Trimof - This is the coolest place for comic books and video games. Norma Comics is there, which is a famous Spanish publisher that sells a lot of original and translater comics. There was also a LOT of small video game stores, specialising in retro/import titles. It's right by the start of Sant Joan, which is a lovely walk up and down the city and goes through many places. There's some lovely parks there too and if the weather is good there's always people to see doing stuff.
If you want beaches Barceloneta is great - as it has a huge long strip of them. Nice places to eat, by the marina and over the other side towards Gothic and Raval you have some amazing bits of history there to just walk around (I wouldn't hang around either late at night as a tourist, but BCN was always safe for me, never had a problem). If you want some other beaches Poblenou is a bit less touristy and has equally nice quieter beaches too and isn't far away.
I loved spending evenings looking over the city in Montjuic, which has amazing views of BCN - usually something going on and at the bottom is Plaza Espana which can get you pretty anywhere around the city either by bus or metro or train.
One thing I would say is do consider doing the touristy things - Park Guell, Sagrada Familia, the Nou Camp, even getting on a tourist bus for a day and jumping off and on when you see things you like. I did it more than once and it's a great way not to just see things but also learn from the guides on the buses on history and you can decide if you want to take some time there or not. Enjoy being a tourist =)
And check out Citymapper - which is a great city travelling app for geting around. I still use it when I work in Paris and it helps with directions and not getting lost, which is easy to do in a city that has a LOT of historical buildings and streets that reflect the size and changes over the years.
It may have changed a lot since I left - but we are going next year for our first family holiday and my first time in nearly seven years in April so I may have more to share then. It's an amazing city which holds a special place in my heart and always will. I hope you enjoy it as much as I did and have an incredible trip there.
If you skipped the above - BCN Is cool, go have fun, be cool to everyone and you will have a wonderful time =)