For most people coming to Barcelona, seeing the numerous Gaudí sites is one of the main reasons for their trip. It´s definitely an acquired taste, and I can´t say I had previously been very interested in modern architecture, but these sites were really interesting and, yes, beautiful. The sites are spread all across Barcelona, but can be seen in a day if you take the subway. The Parc Güell is the largest, and in my opinion, the best of them. A public park on the north side of the city, it features many buildings, figures, statues, and other structures of the whimsical kind that Gaudí is famous for. The most famous building by this crazy genius is definitely the Sagrada Familia, officially called a temple, but really a church - one that will be finished in just a few decades... Apparently not the whole building is of WH value, but only the Nativity facade and the Crypt, which seems to be a bit small-minded by UNESCO...I found it unnecessary to brave the very long lines and to pay the entrance fee, since the good things about this building can really be seen from the outside. The site also comprises a collection of single buildings across central Barcelona, including Casa Mila, Casa Vicens, Casa Batlló, and Palacio Güell (still being renovated). Again, I thought it was sufficient to admire the facades and not see the inside (especially considering Casa Mila´s long lines and steep fees). Definitely worth the 20-minute trip to the suburb of Santa Coloma de Cervelló is the Crypt of Colonia Güell, really a fascinating building and a recent addition to the WH site.
So when you´re in Barcelona, either you´ve gone there to see the Gaudí sites, or you will see them anyway, because there´s really no escaping this man´s legacy there (is there any man who has done more for a city´s tourism industry than Gaudí has for Barcelona? Well, maybe Mozart and Salzburg).