Im able to spend one day in Arles and 1 day in Avignon while passing through. What are some dont miss thigs to do and maybe a great(not too expensive) place to stay in Arles.
Home of the gigantic Palais des Papes and Pont St. Bénezet (AKA Pont d'Avignon). A giant wall still serves as the traditional city borders. Enjoying it: Take advantage of their "pass" system: the more monuments you visit = lower admission prices for monuments. Inquire at the Tourist Office or the Palais des Papes.
Avignon Tourism Office Website - detailed information about all the attractions in the city.
Source of Van Gogh's inspiration. Don't miss the amphithéâtre and the Roman Arena (les Arènes), both of which are still used today. Fortunately, they have bullfights in the arena instead of killing Christians. Enjoying it: Visit the tourist office to get information on the four walking tours, each with a different themes (Van Gogh tour, Roman tour, etc.).
Arles Tourism Office Website
Hope that helps.
In Arles- in addition to the noted stops above the Musee' de Arles Antique is a must see. Beautiful mosaic Roman floors from villas and models of the city in Roman times. In your travels between Arles and Avignon take time to stop at Barbegal. Check steve's books for directions, you might appreciate it more after seeing the model in the museum in Arles, but it is not to be missed if you are a fan of Roman technology.
Another great hotel in Arles is the Hotel De L'Amphitheatre. It's very affordable and comfortable.