El Jem is essentially a one-sight town, but that sight is extraordinary enough to justify a visit on its own. The Roman amphitheatre is one of the best preserved in the world, and wandering its underground passages and empty arena takes a couple of hours at minimum. The archaeological museum a short walk away has fine Roman mosaics recovered from surrounding villa sites. The combination of the two covers most of a morning or afternoon, and the late afternoon light on the amphitheatre's stone arches is particularly good.