Naqsh-e Rustam is just across the highway from Persepolis, and it happened to be the last stop in my long taxi ride from Kerman to Shiraz. I must say that it is one of the most impressive archaeological sites that I have visited. This site contains the tombs of four Achaemenid kings: that of Darius I (Darius the Great), Xerxes I (Ahasuerus), Artaxerxes I, and Darius II. Their tombs are gigantic temples carved into solid rock on the face of a cliff. It is definitely an impressive sight.
The pictures above show the layout of Naqsh-e Rustam: there are the four tombs carved on the cliff, and there is a kaaba-like structure, also known as the Cube of Zoroaster, directly facing the tombs. There are also giant mural reliefs on the cliff walls, depicting monarchs of the Sassanid Period.
You could also see some people visiting the site, so it provides a scale as to how big these rock carvings are. It definitely is a sight to behold. Most people visit it in conjunction with a visit to Persepolis, since it is just across the highway. I absolutely recommend going here if you're a history fan.