Palau Güell is an early Gaudí masterpiece, designed for his longtime benefactor, Eusebi Güell. The mansion, one of Gaudí's first big projects, was recently restored and re-opened to the public. The centerpiece of the building is the amazing ceiling/skylight and a rooftop full of chimnies. Palau Güell is worth the visit if only to see some of Gaudí's early genius, especially his interest in turning something functional (a chimney!) into a magical work of art.
Though less popular than Gaudí's more famous Sagrada Familia, Park Güell, and Casa Mila, prepare to wait in line. When I visited, tickets were sold for entrance every fifteen minutes, seemingly based on capacity. Buy your ticket at the window before joining the queue. Audioguides are included in the price so don't forget to pick one up on your way in.