I have to disagree with Lowden on this, Barcelona is one of the most beautiful cities I have visited, although admittedly I havent yet been to Florence or Venice.
I went nearly two years ago and loved every minute of it.
One way you can get to see all the sights is by taking a bus tour round the whole city; they are really frequent and take you round all the main tourist attractions (as well as a few not so well known ones too); its about £20 for a two day pass but well worth it. You can get off the bus at any time and simply wait for the next one to come when you have finished seeing whatever it is you wanted to see. It makes for a great day out sight seeing!
La Ramblas is amazing for its street theatre, local stalls, and cafes where you can watch the world go by. Personally speaking I think you get just as many pickpockets in London and as long as you are vigilant you shouldnt have a problem. There is a problem with begging and they can be quite aggressive but if they dont go away after a polite request or do not accept what ever small offering you have given them, they seem to understand 'F**k off' particularly well.
Gaudi's cathedral is certainly a highlight and once you have seen it you will start to see his influence on architecture right throughout the whole city; you must also go and see the gothic quarter of the city (its apparently the oldest) but its beautiful and full of bars and restaurants and not badly priced either.
If you want a decent meal (and prepared to pay for it) go to the harbour which is at the bottom of La Ramblas and there is a selection of great seafood restaurants where the service is excellent.
Easyjet have a number of excellent hotels in their selection- whatever arrangements you make, enjoy!
Gary