Aside from La Guernica, Gaudi was the second big takeaway I left my 10th grade Spanish class with. There's really nothing like seeing the places and works you studied for so long, in person. Nothing!
My dad really surprised me when he suggested that we hike an off beaten path up this hill (this from the man who doesn't like to rough it). This area of Park Guell (Bunker del Carmel) is free to the public, but since there's no real path, it was pretty secluded - as if it were a secret spot. The hike up wasn't bad, though I'll have to warn you, the hike down can get a little treacherous if you wear shoes without any traction (whoops!). But take that risk - the panoramic view of Barcelona from the top is really like no other.