Avignon is lovely, although we went one January to escape the winter weather (thinking, you see, of Katharine L Oldmeadow's "Princess Prunella"), only to find that it snowed! But in May it should be glorious. And you can get a bus or train (either, it's the same ticket) to Arles for one day of your holiday, too, and there's a lovely ex-Carthusian monastery to visit across the river....