If you arrive a bit early to Sagrada Familia and can't enter, it's worth the time to quickly stop…read morein here. The beauty is not only browsing through the merch, but to also visit the museum to learn a little history about the team, stadium, and the partnership they have with the UNHCR to see the work they've done to assist refugees. It was quite an interesting experience that you don't normally see in these megastores for other sports teams.
Note that if you're actually a fan of FCB, you can pay extra to get a photo with the championship cup. I passed, but I commend the staff's effort for convincing others otherwise.