Starting and ending in Nice is what most people will do since it is an easy airport to navigate. Nice could easily be my favourite European destination offering both a beach and the city. It deserves a couple of days to explore this beautiful place.
I would highly recommend you rent a villa, if you choose to visit in September, a pool may not be a necessity but a heated pool will be a luxury. Try to choose a place that is walking distance to town for the early riser who may want to pick up coffee and croissants. A town with restaurants can come in handy after a day of travelling but plan to reserve at least a day in advance to avoid disappoint of a full house. Make note that most restaurants close through out the day around 2;30, making a late lunch difficult. Some establishments will be closed on Sunday and Monday.
One hill town that is worth seeing is Seillan, a quaint place with a fabulous restaurant called Chez Hugo on 4 rue de l'Hospice. People are friendly but it certainly is handy to know key terms in French like: Please, Thank you, A bottle of water or wine and The number of people in your dinner party.
A smile will always work too!
I loved St Paul De Vince with it's many alley ways full of unique artists and their creations. A day wandering in and out of shops is best spent in this city. A nice little restaurant just outside the wall is La Petite Chapelle, they have a reasonable Plat du jour and a pleasant terrace. The time of year would greatly effect your overall experience since bus loads of people visit in the summer months.
While visiting Monaco try to park in the first available parking garage, wear comfortable shoes and spend the day exploring. The casino and palace are on opposite sides of this small city but easy to manage with the availability of city buses as well as hop on hop off buses. A nice sea view restaurant on J.F. Kennedy is Miramar, it has a fantastic view of the many yachts. The pure luxury of the place is undeniable with high end stores and every desirable car you can imagine.
Time is the only limit you have in southern France. Enjoy!