One week Nice to Arles for first week in May.

Cassis is on the way to Arles via train. I would stop and see this charming town and do a tour of the Calanques from the port. Staying a night would be ideal. The Calanques are Provence's version of fjords. Note that the train station in Cassis is not downtown. You will have to catch the bus just outside the station to get downtown. It is a short walk to the port from there.

There are many "highlights" in that region, which is rather large, so with only a week and with no car, you will have to be very selective, depending on your interests. I'd start with the book Rick Steves Provence And The French Riviera; note that his general France book has much less coverage of this area.

I saw this as two separate trips, each with no car. On the first (in autumn), I stayed two nights in Marseille (I liked it, others don't), three nights in Arles (very nice) with a day trip to Nimes (my favorite of all, with a GREAT Roman arena), three nights in Avignon (good but not great) with day trip to Orange (the town is grungy, but the Roman theater with its intact theatrical wall was thrilling), and two nights in Aix-en-Provence (just OK; I don't get the appeal). Without a car, there are some other places in the area you can see, but lots of other places (small villages, etc) will not be easy to access.

On my second trip (in spring) I spent 10 nights in Nice, which makes a very convenient base for trips by bus and train to the entire Cote d'Azur region. I saw Villa Ephrussi, Monaco (which I liked more than Rick does), Menton, Cannes, Juan-les-Pins (which I liked more than most of the other towns), Eze Village, St. Paul de Vence, and Haut-des-Cagnes (Renoir's house and a very nice town to boot). And that's just what I remember without looking at my notes. From Nice, you have your pick of these and many more, and no car is needed.

I wish Terry Kathryn would see this post and respond, she is very knowledgeable about this area. Or you can find one of her posts in General Europe under "Movies..." and send her a PM.

Two caveats, based on our trip last May. First of all, if you are planning to visit the calanques, you need a Plan B. We were rained out one day. The next day the boats didn't go out because of windy weather and choppy water. Second, plan carefully around the May 1 holiday. We were in Nice that day. NO public transport was running, which was sort of immaterial; because ALL the museums and many restaurants were closed.

I agree with a lot of what Harold has said about the various towns. (Haven't been to Monaco or Juan-les-Pins.) We also had a car, so I can't speak to public transport access. However, we liked Rousillon a lot.

I had a guide one morning in Arles, who was listed in one of the editions of RS' Provence guide named Alice Vallat,
who was very kind and as familiar with the sights as the proverbial back of one's hand.
mailto:alice.vallat@voila.fr
She likes talking impressionism, but had no trouble weighting our walk towards my own interests in the Roman period.
If you do like ancient stuff, the museum there in Arles is departement-level quality and well worth a long visit.
Note that the back-door neighborhood between the museum and the historic core that Rick mentions has now
been pretty thoroughly discovered -- reminded me too much of that street in Madrid leading between the Prado and the central plaza that is lined with overpriced cafes. I think the non-Anglos these days tend to stroll the riverfront in Arles and shrug their shoulders at the cruise barges.
Near the river, one of Rick's recommended hotels is the Hotel du Musee, which has great character and warm staff. If there's one consistent thread that I find with Rick's recommended hotels, it's that when I chat with their proprietors and get to know and be known a little bit, they unfailingly send me to the right restaurant for me*. I always have a bias in favor of cheap accommodations, but I have found in the last 8 years or so that when I loosen my purse strings slightly and spring for one of Rick's recommendations, the advice I get from the proprietors more than makes up for the additional lodging expense.

My tip for Nice is that the fine arts museum is well worth the small extra effort of getting there, but the Asian art museum not so much (and I have some background in Asian art).

*In Arles, the right restaurant for me was L'Ecrin

https://www.facebook.com/pages/Restaurant-Lecrin/355239264611649?fref=pb&hc_location=profile_browser&rf=672700636106086

This is an area where having a car gives you the flexibility to go where you want, when you want.