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It's not walkable. Take a taxi and share with 3 others, make sure they put the meter on. Depends what the cruise is charging but it is fairly easy to do it yourself.
Book in advance though because it does get full. You can book on this link:
This link is for Angie's Taxi's who get good cruise reviews, book your taxi in advance as well so you are not hanging around:
The hop on hop off tourist bus, northern circuit, goes there....alot of people on our bus got off there to do a tour of the stadium...Camp Nou is north of the city and it is too far to walk. The tourist bus was 19 euros each for the day last year and also went to the Sagrada Familia..Gaudi's church.
You can getto the stadium when you use the tourist bus and it costs around 20 euro for the bus and you can hop on and off where ever you like.google barcelona and see what you find.