Gaudi simply created a breathtaking, awe inspiring masterpiece in his construction of the Church of the Sacred Family. For less than 15 Euro, you can tour the entire site and the price of entry is more than worth it. The building is still under construction, it has been for over 130 years! Upon arrival, it is easy to understand why as no detail has been spared in creating this architectural gem. Many have seen this attraction in pictures or films, but it takes being there in person to truly appreciate how other worldly this building is. The grand ornately decorated, expansive spaces yield to impossibly tall sky scraping towers and it is clear that Gaudi set out to create something truly memorable, and that he did. When in Barcelona, this is without question, a must see attraction. I recommend starting here, setting a great tone for the rest of your explorations in Barcelona.