I have visited quite a few cities in Europe and Barcelona is the best, it offers you amazing value for money. This post is about lads holidays but a lot of it can be applied to family or business holidays as well.
Here are my tips-
Book early and book off season like early March.
Book flights and hotel separately.
Book apartment or hotel directly on their website as you get better deals.
If you are travelling light then you can even use metro to travel from airport to the hotel (not an option with the family).
Don't hire car through Ryanair (scamsters) and Goldcar (bigger scamsters), hire at the airport or use Enterprise/Europcar.
Check for feedback or reviews before you book anything, it goes for the services you might want to use in Barcelona ;)
Must visit Montserrat.
Visit Gerona/Costa Brava
Visit Tarragona if possible
Don't shop in Rambla, shop around and check Gothic quarters.
Don't drink tap water.
Buy a family ticket or t-journeys for Metro.
Visit Olympic village, day/night both.