Arles is a must visit if you are in Avignon and you are interested in the history of France during the medieval time. The city holds plenty of historical landmarks from the roman period like the coliseum and the necropolis.
Not to mention, every streets in Arles are photo-worthy. Yes, every one of them! Check out the photos if you don't believe me but the streets of Arles are the kind of streets I expect to see in France. It just fits so well with the vintage and romantic perception people have about France.