Bring lots of money, as Randy said.
September is a good time as the French school holidays have finished and the traffic will revert to just madness instead of insanity. The roads resemble waiting lines for car-parks that are tens of kilometers away in season.
St Tropez is a nice spot to visit late afternoon, as the sun sets on the port in the evening (the only spot on the Riviera with the setting sun. The beaches around the corner have sand, and there is a different section of beach for the different 'interest' groups. However, if you are going to use the beachside cafes (or the places in the Port), as Randy says, have plenty of money.
One restaurant that I visited a couple of times about 9 years ago is Bruno's (just Google Bruno, and he will appear). It is about an hours drive inland from St Tropez (which is an hour or so west of Nice). A magnificent degustation menu with an emphasis on truffles and exquisite cooking. Value for money. Not cheap but not expensive. I hope it is the same. If Bruno is a 6'6" giant of a man, it is the same place. Don't plan on doing much that evening after lunch, and allow for a slowish next morning as well.
Have fun, the sun and light make this part of the world a magnificant place. There is a reason there are so many people there.
Can't tell you about the golf. There is a course that climbs the hill behind St Tropez, but I am not recommending it to you. Very hilly, and narrow in parts. I have only seen cart golf there.
James B