Església Parroquial de Sant Nicolau (Church of Saint Nicolas)

While you should definitely go to the main Cathedral of Valencia (and climb the bell tower if you aren't afraid of heights), this is a smaller church in the same Old Town area, sometimes called "the Sistine Chapel of Valencia" because the walls and entire ceiling are covered with amazing frescoes. The audio tour is well worth doing — it walks you through the story in each fresco.