Any cross body bag will do. Just remember to keep your bag on your knees when eating in a restaurant (never leave it on the floor or chair). Be careful when getting in and out of metro. You don't need to put your money in your shoes or bra! In this site, you'll find some of the most common techniques, but read it just to be prepared and not scared. jon.es/barna/scams.html Barcelona is an amazing city, and you'll have a great time. Don't let your fear rob your pleasure!
I was in Barcelona in April and had zero problems with my Kipling crossbody bag. The strap is super thick and would take a machete (which, luckily, I didn't see anyone walking down the street with....) to cut.
Barcelona is fab! Don't let the tales of heavy petty crime ruin your trip--it's really not that bad if you're smart about it.
I got my purse nicked on Train to Barcelona - was struggling to get on with 2 DDs (14ish months and 6) while helping DP on with buggy. Really busy train. 10 minutes int journey noticed some bastard had opened my bag and nicked purse - had about 100E in it. Cancelled credit cards on train, but had to wait until I gt off train and fund internet cafe to cancel maestro card - by the time I di that they'd managed t get £350 out of it (think they'd been watching me take out money at train station). Bought new cross body bag, with flap and zip and didn't carry cash.
Be careful of the gypsy women with kids if you are sitting outside eating - one nearly had my purse, but I got to it before she did.
Having said that - Barcelona is my favourite city in the world, enjoy your trip