I've been like 5 times now and it's a bloody wonderful city. The Metro is great to get around but as long as you're not an idiot you should be able to avoid the pickpockets.
Las Ramblas is definitely not the place to eat or drink. Walk about 10 minutes in any direction and you'll be able to find food & drink that's much, much cheaper and much, much better.
So as far as places to eat, there's looooooooads of options depending on whatever you're in to, so here are a few places you should consider.
Gringa - In Raval/Poble Sec neighbourhood. Does some of the tastiest fried chicken you can get. Incredible hangover scran and has been a post Primavera Sound staple for me for years now.
See also: Gringa All Day - Like a 10 minute walk from Plaza Cataluña and does superb breakfast/brunch scran. My personal favourite is the fried chicken and waffles with a michelada to drink.
Narciso - There's 2 branches of this. One of which is just round the corner from the cathedral in the Gothic Quarter. The wine is great and the Colloseo sandwich is a masterpiece. Highly recommend.
Restaurante Casa Mari y Rufo - Next to Mercat de Santa Caterina and is a tiny wee hole in the wall place we stumbled across by accident. Incredible, incredible seafood done so simply. Had to really push the boundaries of my Duolingo Spanish in here because only one of the waiters spoke a little bit of English and the whole menu is Spanish.
La Pepita - I bloody love this place. The food is outstanding. Worth a visit if you're looking for food that's a bit fancier but in still a relatively casual setting.
El Xampanyet - Good wine, good tapas. What's not to love.
Sightseeing wise if you're going to go to Sagrada Familia it's really cool inside. Pre book your tickets and go early though. Saves a lot of faffing about. La Boqueria is quite cool to wander around but it's alway rammed so, again, mind yourself for pickpockets.
Parc Güell is cool and has a great view of the city. You can pay to go in to the centre of it but I wouldn't say it's strictly necessary. Montjuïc Castle is really cool for that too. You can go up in the cable car and see right across the whole city.
Depending on how long you're there for you can jump on the train up to Girona for a day trip. The old town is like 3000 years old in parts and it's a beauuuuutiful place to have a walk around.