Mr B is a football fan so when I asked him one thing he wanted to do, he said was to go see the grounds of FC Barcelona. He is not a fan of the club but they are an impressive team. The cheapest tickets I could find were on the TripAdvisor website, just a warning, you do have to collect the tickets as the confirmation of the email is not enough. They have set times that you can tour and if a match is on the place closes early as the museum is also on the football grounds.
While we were there, FC Barcelona had a match on so Mr B went to a game. He said that the atmosphere was amazing. So highly recommend it if there is one on while you are there. You can buy them online or the tourist information desk is an authorized seller of tickets.