OK, hope this helps:
bars/food
my fave food in BCN is Xampanyet on Carrer de Montcada, 22. It opens at 7pm with v limited seating. Get there at 6.30. the queue is worth it. the food/vibe is sensational (it's usually shut on sundays though)
best bar is Bar Marsella (for the absinthe/history), I really like Beirut too, also Betty Ford's, Fleadonia (you have to pay €3 on door to become a member but worth it). The real cool shit is around Born. Avoid the Ramblas - seriously horrible nowadays. if you venture out of the centre, there are thousands of cool bars in the Gracia area too - the alternative end of town which can get a bit 'crustie' but is generally a lot more interesting.
tourism
forget parc guell unless you book well in advance. it eats into your time and is overrun by tourists (see also sagrada familia), check out the geese in the cathedral, gaudi's houses on passeig de gracia are pretty cool but you'll have to queue. Culture-wise worth checking the CCCB and MACBA for exhibitions. If weather's good, get the tram up to Tibidabo or cable car up to Montjuic - views of city are really cool. extra points if you're with a love interest.. if you're around during the mardi gras, get the train to Sitges, about 40 mins south.
Nou Camp
museum's ok but I'd suggest only going to Nou Camp if there's a game on and even then only if some no-hoper side are visiting (like Granada or Leganes), otherwise it will be extortionate, tix will be inflated (legit or otherwise) and you may waste your time if you can't get in.
clubbing
check the barcelona threads already on here. i'm too old for razzmatazz and too antagonistic to the El Row or Opium experience but they are probably the biggest ones in town. I personally prefer Sala Apolo (for NITSA), MOOG, Macarena (great music but it is tiny) - there's also a new-ish night called Origins I need to check out too aimed at a more mature clientele & focusing on the 'future disco' side of things + the fortnightly Glove Party is really good fun too. if you just want convenient cheese then Karma on Placa Reial caters to your every need as do the various discos on calle aribau and the eixample area.
live music
sidecar, sala bikini
record stores
Discos Paradiso (top vinyl selection + loads of info there about one-off events etc)
general tips
watch bags/pockets at ALL times, when in barri gotic/raval. If you buy beer on street, remember to clean rim, it will have been stored in a nearby sewer! (they will almost certainly offer other things too - exercise discretion). If you need the cops, the best comisaria is on Carrer Nou de la Rambla.