Park Güell
It is one of the UNESCO World Heritage Sites in Barcelona and by far my favourite place in Barcelona (haven’t visited all places so it’s just a tentative conclusion). It a big park decorated with many interesting architectural elements designed by Antoni Gaudi, built from 1900-1914. Many of his works resemble the form of nature, e.g sea waves, tree trunks, shells, leaves, branches. The best thing is, it’s free! No entrance fee except the little Gaudi Museum.