The Arènes de Nîmes, built around 100 AD, is among the most intact Roman amphitheatres anywhere. The complete outer ring of two-tier arcades — 60 each — still stands, the seating tiers inside hold 24,000 spectators, and the building is still used for concerts, opera, and the city's twice-yearly bullfighting feria.
Visit during a quiet morning if you want to see the structure; the audio guide does a careful job explaining how Roman crowd management worked. Better still, get a ticket for an evening event — opera under the stars in May, the Feria in September, a Sting concert in July. Standing in the same stones the Romans cut, watching a performance the way the Romans did, is the single best thing to do in Nîmes.