St tropez

You do realize that even with a private driver it will be about 2 hrs in each direction from Marseille, and it will be quite expensive. By public transport it will be closer to 4 hrs each way from Marseille. Was there something in particular that draws you to Saint-Tropez?

Are their sites that make it worth that hassle?

Seriously I have never been and mostly done planning a trip to the region for this July. Thought was to skip St. Tropez entirely even though I have more time than you do just cause it seemed like there was not much there other than nice beaches, expensive yachts and some high priced shopping. Saying this partially tongue in cheek, I am sure it is beautiful but it seems like much of the area is also beautiful so one has to prioritize what is most important to them and not much was drawing me to Saint Tropez other than it's name/reputation. Add in the location is a bit out of the way and traffic there is said to be bad and it had me questioning it.

I also opted to skip St-Tropez during my 2017 visit to France, despite spending a lot of time in the south. When your time is limited--and if you're measuring it in hours, it is very limited--I highly recommend taking logistics into account when selecting destinations. There is no shortage of picturesque small towns in southern France, and a lot of them are on the water if that's important to you. St. Tropez doesn't have rail service, so the route from Marseille by public transportation is extremely convoluted; basically, you'd probably have to turn around before you even got there, and that's without figuring in the time it will take you to get to the Marseille train station to being the trip.

Edited to add: The driving route is 90 miles, so even by taxi it would be a long trip. I don't even want to think about the cost.

Unless there is something very specific drawing you to St-Tropez, I suggest going to Cassis instead. You can get there in less than 25 minutes from the Marseille-St. Charles railroad station, but there's quite a walk from the Cassis RR station down to the picturesque port. You'd need to Google for information on bus service (which does exist), or perhaps someone here knows whether there are usually taxis waiting. The walk to the port is downhill, at least. If you manage the trip down to the port efficiently, you might even have enough time in Cassis (which is quite small) to take a boat ride out to see the Calanques. If you're on a cruise, that might not be your first choice.

I also liked Hyeres a lot. It's not on the coast, though, and the ride from Marseille is about 1-1/2 hours. I'd be nervous trying to go that far if I had to be back on a cruise by a specific time.

While there are a few historic sights, you can do a fly by and see most everything in the main part of the town, the harbor, etc. Likewise, unless you are passing by near, I would not divert away from your journey. It's a resort to enjoy the dining, the beaches, the yachts in the harbor, sample Provencal rose, and the luxurious accommodation, not to see some museum or ruin. Near Marseilles? Go to Cassis instead -- while certainly small and less luxe, has the Calanques (but you will need more time). (eta - posted the same time as above answer)

Speaking from experience, the town of Saint Tropez in August is a huge tourist traffic jam after 9 am. You'll spend much of your valuable time sitting behind camper vans trying to get to the port/old city area. I concur with the suggestions of other contributors to go elsewhere. When I have stayed near the hill town of Ramatuelle for the August theater festival, I dreaded having to go into Saint Tropez. I don't normally mind crowds, but this was madness.

I agree with Barbra(above)we go down there every August for our Holidays and the Traffic in and out is a nightmare during this period.You could easily spend 1 hour in and the same out just traveling 2 miles on one of the 2 routes into the town which are always jammed.
I would skip this idea unless you have a full day and arrive around 9am before the mad rush.