Barcelona is a great city to visit, and in recent years has overtaken such European cities as Venice and Berlin as a top destination. If you've visited or live here, then you'll know what all the fuss is about, but if not, then it's wise to have a few ideas of Do's and Don'ts with regards the city.
DO visit the city's stunning architecture and great museums. Barcelona was awarded 9 UNESCO World Heritage Sites - 8 of which are in Barcelona (the 9th is Colonia Güell, just outside the city) - and all are worth a visit, even just from the outside, sitting on the tourist bus, for example. There are some great museums, too, such as the Picasso Museum, the Erotic museum (!) on las ramblas, and the Chocolate Museum. Make sure you visit these places, but....
DON'T turn up in beachwear, or inappropriately dressed. I don't mean as these are religious places, so you need to cover up, but it's a real pet hate of the locals to see guys in trunks and flip flops with no tops, or Girls in bikini tops and a "pareo" waltzing along las Ramblas, or even worse stopping into Carrefour or Corte Ingles. Keep the beachwear for the beach, guys.
If you travel by car to Barcelona, DON'T leave your valuables in the car when you street park. I know of some people who parked right in front of the restaurant they were eating at in Barceloneta, and even sat outside, almost in full view of thier car. When they returned to the vehicle, the back window had been smashed, and they lost a mobile phone that had been left in the back, and then had the annoying episode of having to report the incident to the police for their hire car company.
DO hydrate yourself around Barcelona, especially the young ones in Barcelona. I could restrict this to the summer, but often temepratures are just as great in April and late september, so use this rule as a norm.
DON'T fall for the street scams - the dancing dollies (see my recent post here ), the 3 cups game, and the Barcelona dance. These are all easy to see, and if you watch the beer intake you should be ok!
DO visit the Boqueria market on Las Ramblas to try the "pata negra" Iberic ham (black hooved ham). It's one of the best, and is fed only on acorns. They also have some chorizos of the same animal for you to either try there, or take back to your Barcelona apartment to prepare. However, ...
DON'T stand on street corners with a big map, announcing the fact that you're lost! Try and find a phone booth, or a well lit shop entrance to find your bearings, and don't be afraid to ask. Things have got a lot easier recently with the installation of map-posts and directional arrows, to help you along your way.
For more information on Barcelona, see this Barcelona Guide.