Agra's monuments are mostly Mughal and mostly within a 10-kilometre square either side of the Yamuna river. The five biggest — Taj Mahal, Agra Fort, Itimad-ud-Daulah, Akbar's Tomb at Sikandra, and Fatehpur Sikri — would fill two long days. Smaller monuments (Mehtab Bagh, Chini ka Rauza, the Tomb of Mariam, John Hessing's Tomb) reward a third. A combined entrance ticket gives a small discount on the major sites.
Most monuments are closed on Fridays except the Taj Mahal, which is closed only on Fridays. Plan accordingly. Sunrise at the Taj is the highlight of most visits; book your ticket online to skip the morning queue.