Last year, my wife and I had less time (8 nights) in the same area as part of a longer trip. We started in Nice, went on to Lacoste, and then to Nimes.
Lacoste was a great base for exploring the Luberon, and a quiet respite after the bustle of Nice. We rented a great apartment that had a terrace where we enjoyed food and wine from local markets and vineyards with a view across the valley of Bonnieux. There weren't many restaurants, but we had a car and it gave us a chance to see other villages.
We also enjoyed Nimes, where we had another great apartment near the arena. Nimes is smaller and less touristy than Arles, where we had spent the day along the way. If you enjoy Roman ruins, Nimes has the Castellum and Maison Carrée, along with the aforementioned arena which was spectacular. Moreover, with a car we did the Pont du Gard and Orange with the Triumphal Arch and Theatre. With more time in Nimes we would like to have explored Uzes nearby.
If you plan to stay in Arles, I'd suggest going to Nimes as a day trip rather than including it as a base. We only had dinner in Roussillon but it seemed nice, and like Lacoste, would be a good base for exploring the Luberon.
Have fun!