Hold onto your wallets/bags when in the busy part of the city.
I had two friends who had things lifted in one day! (they weren't even traveling together). Barcelona is a busy place, with some people who are very good at lifting things from unsuspecting tourists!!
Well, I'm glad poths got the warnings before he went.
To think that fun is simple fun, while earnest things are earnest, proves all too plain that neither one thou truthfully discernest.
A friend of mine always carries a cheap decoy wallet with an expired and canceled credit card and credit card-looking things like Subway and Safeway cards, some low-value currency, (from the last country he was in if that's applicable), etc. He uses a money belt for his passport, cash, real credit card, etc.
"The team doesn't get automatic capacity because management is mad" -- Greg Smith, agile guy