seriously, be very careful round the red light area.
Barcelona in general is very high for street crime - usually non violent - bag snatches, pick-pockets etc.
i'm 43, and lost a bag there last summer on a 2 day trip. Saw over a dozen other english couples in the police station in an hour reporting thefts.
There's some great bars, and the beach is good too, but if you wander off drunk you'll have problems.
Honestly, I go to football, home and abroad, am clued up, and have travelled a fair bit, but found Barcelona and it's narrow streets in the Old Town a bit hairy.