The Musée de la Romanité, opened in 2018 directly opposite the Arena, holds the city's collection of Roman antiquities — mosaics, sculpture, funerary stelae, household objects, and a partial reconstruction of the Maison Carrée's frieze. It is one of the best Roman museums in France and the obvious complement to the city's monuments themselves.
The building is itself a contemporary landmark, with a rooftop terrace giving a striking aerial view of the Arena across the street. Allow two hours for the collection, more if the audio guide draws you in. Combined tickets with the Arena, Maison Carrée, and Tour Magne are excellent value.