Me and the wife went there few years back, so we did turisty-wifey stuff, like visiting architech Antonius Gaudi's main pieces of work: Parc Guell (nice views of the city, trippy architecture), Sagrada Familia church (huge!), and La Pedrera (meh...). The subway takes you anywhere.
We stayed in the heart of the city, La Rambla, which is the place to be for good eats and really cool bars. Spain makes some really good beer, try San Miguel. People are very receptive, outgoing. Good looking girls too. The food is some of the best I've had...you've got to have a paella when in Barcelona, this is the real deal, not the crap you'll find at some latino restaurant down the street.
And....I tried absinthe :nuts: at this smokey jazz bar where supposedly Hemmingway used to hang out. It wasn't what it's all cracked up to be, didn't do nothing for me really, just a lil headache the next day. But the bar scene is great, and they'll serve afterhours Tapas anytime you like.
Barcelona is easily one of the best cities I've been to. I imagine a single dude will have a blast there.