We discussed Barcelona about a year ago - http://www.wordmagazine.co.uk/content/holiday-destinations-barcelona
but that needn't discourage any more up-to-date suggestions.

I loved it. My main suggestions on the earlier thread being

Must-dos are the Picasso Museum, Montjuic, Sagrada Familia, Parc Guell, the street entertainers on La Ramblas, the beaches, and the flea market at Plaça de les Glories Catalanes

Hotels are expensive, but this place was fine when I visited a few years ago http://www.forkandpillow.com/es/hoteles/hotel-market/ (though it seems to have been taken over since).

Oh, and make sure you know where your purse or wallet is at all times.

It's lovely, but do bear in mind that everyone I know who's been recently has been mugged or at least knows someone who has.

My colleague was mugged on the train as it pulled into the city on the day she arrived. They took her money and passport. She did not have a great weekend.

Las Ramblas is a notorious mugging ground. My wife went with four work colleagues and three of them were pickpocketed.

Sorry, I hope this doesn't take the shine off it, but you do have to be careful.

You haven't book anything yet I hope! Madrid's far nicer.

Parc Guell is wonderful, but go first thing in the morning (I think it opens at 8.00), it gets far too crowded within a few hours.

If you're of an arty bent, the Barcelona art card costs about 30 euros (IIRC) for a week and gets you free entry into all the major galleries & museums, amazing value if you're prepared to cane it.

Obviously it might not fit in with your birthday trip theme, but a visit at the end of September to coincide with the La Merce festival means you'll get loads of free gigs, firework displays on the beach and events like the "fire run"... it's worth Googling.

... and I've been there for holidays 5 out of the last 8 years and have never been mugged or pocket-picked, don't be too paranoid (but be careful of course.)

Not much to add, but suggest you see it in the evening, as it looks nicer when it's lit up. Also, arrive via the metro. If you leave by the right exit the first thing you will see is it towering above you. It's like that scene with the castle at the end ofTime Bandits.

Oh, and when you get to the end of the Ramblas and see the statue of Columbus, don't forget to point out to Mr H that Columbus is actually pointing towards Italy, rather than the US, meaning that the statue is almost as clueless about geography as the real thing was.

and i work in the centre and it's way safer than 99% of other major cities.
Just be sensible with your bags on public transport and the main tourist areas.
The tourist bus is very popular and takes you everywhere you need to go. Public transport has improved tenfold in the last ten years and if you need it,pm me and i'll give you the number of an English speaking cab driver.
Tickets for the football are easy to get,except against Real Madrid.Any "La Caixa" ATM or book them online for Barça,Expect to pay Premier League prices or experience the Mighty Espanyol,cheaper and a much better atmosphere.
restaurants vary from expensive to reasonably cheap,
http://www.cankenji.com/
is our next door neighbour's restaurant
always packed.

http://fundaciomiro-bcn.org/?idioma=2

I took mrs rob for the weekend and we stayed at Hotel Balmes which was fantastic and not expensive. We missed all the art and just ate and drank and wandered and swam in the sea.

is my favourite building in the world & a must-visit!

Do take the tourist buses - you get to see everything and can stop off at will.
Botafumeiro is a fantastic seafood resto, but expensive.

for all the brilliant suggestions so far. Maybe Madrid next year then?