Jerash is a must-see. Hidden under a Circassian farming town until excavations began in the 1920s, the ruins of ancient Gerasa turned out to be among the best-preserved Greco-Roman provincial cities in the world — sometimes called the "Pompeii of the East."
Hundreds of columns still stand along the colonnaded streets, flanked by temples, theatres, baths, and churches spanning the 1st to the 7th century. The Oval Plaza, ringed by 56 Ionic columns, is the centrepiece; the South Theatre has extraordinary acoustics; and the Temple of Artemis, perched on a hillside overlooking the whole city, is grand even in ruin. The Cardo Maximus — the main colonnaded street, 800 metres of original paving still in place — connects the city's major monuments. The Arch of Hadrian at the southern entrance and the Hippodrome beside it (where daily chariot demonstrations are staged) set the scene before you even reach the main site. In summer, a sound-and-light show brings the site to life after dark.
Jerash is less than 50 kilometres from Amman and is easily visited as a day trip. Allow at least half a day — there is far more here than can be absorbed in a quick pass. The Jerash Archaeological Museum inside the site is worth an hour, and the Jerash Cathedral gives a vivid sense of how thoroughly the city's fortunes changed from pagan Rome to Christian Byzantium.