Because it is a small town, but still near major train routes, Arles is a great location to day trip from. Some of my favorite places to day trip in Southern France include Avignon, Orange, Nime and Pont du Gard.
Avignon has a great medieval wall that circles the old town and what was once the Papal Palace. The Pope moved here from Rome when there was too much turmoil in Italy.
Orange is famous for it's Roman Theatre, which is the best preserved anywhere.
I was not as fond of Nime as the places I listed because it is more of a modern city, but it does have a large Roman Arena. Mostly I day-tripped there so I could catch a bus to the Pont du gard, a famous part of a Roman Aqueduct. Not only is the Pont du gard a bridge across a ravine, there is also a tunnel through the hill side which once carried water, you can walk through.