I did a Friday - Monday with my mate a few years back & stayed here
http://hotelzenitbarcelona.zenithoteles.com/
Was pretty central, 5 min walk to one of the main roads leading to las ramblas, nice rooms & reasonable prices. I recommended it to another pal last year and he had no complaints either.
Nou Camp is superb, although when we went Barca lost 3-1 to Athletico Madrid (Larsson scored Barca's goal, as if I hadn't seen enough of him scoring!)
Sagrada Famillia is a must, stunning building inside & outside.
Hope that's some use mate.