If you are ever going to Barcelona it is a must that you check out Gaudi's Modernisme architecture. It is some of the most whimsical and organic architecture I have ever seen. Park Guell is where Gaudi turned to landscape, the winding paths will take you up hills and points where you can see the whole city. There is also a cafe where you can sit and have a beer or sangria and take in the colourful nature around you. Definitely one of my favorite places from my trip. As well it would do you well to see the Sagrada Familia, the unfinished church in Barcelona, you can see the construction and the two obvious styles, where they start and end. It really is a beautiful mash up of old and new. Your money will be well spent to check out Casa Batllo as well, a house which Gaudi had renovated. It is absolutely stunning, winding staircases and huge arches which make you feel as if you are inside a whale's skeleton! Even the exterior has a boney-beastly appearance. The whole house is hinted with stained glass as well. If you go to the roof top you will be able to see twisting mosaic forms, as well as a great view of all rooftops in the area.
I did some research before I left for my trip on architecture and Surrealism, and I can say that the Modernisme architecture throughout Barcelona ties in very closely to what the Surrealists looked for in architecture, space and mood. As well as the importance of contrasting light and dark, straight and curving. You really realize the impact of architecture in a city as I felt very happy, uplifted and even colourful as my surroundings filled me with delight and amazement.