Sunday/Monday in Provence

If you're interested in the art museums in the area, you need to check the website of each one you might visit. September is not peak season, and both days and hours of operation may be cut back. I'd do the same for other key sights.

I haven't been to Nimes, but I don't think you'll have any difficulty finding food. Avignon in particular seemed well supplied with places to eat. A lot of restaurants keep rather limited hours, however.

September isn't the peak of summer holidays but not low season, either. Google Maps may possibly have more opening time info that a restaurant's own web site, but take both with a grain of salt.

Nimes is a bit strange in that many restaurants are only open for lunch. Of those open past 19:00, some will also be closed Sunday and/or Monday. But you'll find something. Those from Rick's guidebook that avoid both issues (in September) are Le Lisita, L'Imprevu, and Restaurant les Magnolias.

In Avignon, you at least don't have the early closing issue. Rick's listings that serve dinner daily include L'Epicerie, La Vache a Carreaux, and Bistrot a Tartines.

Sunday in Avignon was DEAD. It seemed like every restaurant was closed, along with most shops. I imagine it might be similar elsewhere, so plan accordingly. Monday everyone was back at full tilt.

Maybe do Pont du Garde on Sunday, and just pack a picnic.

Sunday is market day in L'Isle sur la Sorgue. It's an entertaining eyeful: antiques, merchandise, plenty to eat from the vendors and some, if not all, of the local restaurants are open as well. If you're interested, this link has photos and more info. https://independenttravelcats.com/2013/12/06/maket-days-in-provence-visiting-lisle-sur-la-sorgue-sunday-market/

(Sorgue is a pretty 40 minute drive from St. Remy; also 40 minutes from Avignon. Not sure what your base is, or whether you plan to relocate 4 times.)

We stayed one night in Avignon and rented a farm house in St. Remy for six nights, getting up every day and heading in a different direction. We arrived at the Avignon TGV station late afternoon (you didn't say where you are arriving from) and took a taxi into the city. The only things we saw the next day were the Palace of the Popes and the Avignon bridge (or half of it), before going back to the TGV and picking up our rental car (don't drive in Avignon). From there we drove to the Pont du Gard, one of the must see highlights, and on to Nimes before finishing up in St. Remy. At the Pont du Gard be sure to walk across it, then take the path around to the right for the best pictures. Looking at your limited itinerary with only four days, personally I'd leave out Avignon and Aix (boring), opting to see other towns such as Nimes, Arles, Cassis on the Med (take a boat ride) and as someone else suggested, L'Isle sur la Sorgue. A delightful town and markets are not to be missed in ANY of them (markets move daily, but you can find a list of where they are and when). We also went to Orange north of Avignon and a hill town called Gordes (pronounced Gord). The Russell Crowe movie "A Good Year" was filmed in and around Gordes and its a great movie to get you in the Provence mood. France is easy to drive in and distances are short. Keep a bag of Euro coins in your car for tolls and watch your speed.....cameras everywhere. As for restaurants, we had no problem in any of the towns finding local style places with excellent food, and a variety of menus and prices. If you want any of our itinerary or lodging details please send me a private message.

Jim

Many public sector museums and historic sites in France are closed on Mondays. A few close on Tuesdays instead, but you need to check on the individual website.

A note - if it's mid-September the weekend of 16th/17th is the French national Heritage Days (journees du Patrimoine), when some places that are not usually open to the public are, and other sites have special events. You can find details here. Note that high-profile locations may be very busy and sometimes require prebooking.