Mies Van der Rohe's Montjuïc Pavilion is found on the walk between Plaça d'Espanya and the Magic Fountain, and is constructed of marble, steel and glass. Originally built in 1929 for the Barcelona International Exhibition, it was reassembled in 1983. You can walk around the exterior of the pavilion to appreciate the modern clean lines or view the interior of the Pavilion for a 2,50 Euros entrance fee. The closest metro station is Espanya.
This may seem as tiny insignificant museum to common travelers and would be disappointed to pay 6 euros to enter, but to me Architect, it was the most fascinating architectural masterpiece I visited in Barcelona, and liked it as much as the Sagrada Familia. In fact when you have the knowledge that this oeuvre was built at the same time when women dressed in huge gowns and towering hairstyles, you can't help but appreciate the minimalism of this place. As Mies says: God is in the Detail.
Pretty cool building but you probably have to visit on a sunny day to get the full effect.