French Riviera

I wouldn't go all the way to St-Tropez for a beach because it has no train station. You'd have to take the train to St-Raphael and switch to a bus or taxi--and it would be neither a short nor an inexpensive taxi ride. There may be ferry service from elsewhere along the coast, but again not a quick journey. There must surely be easier options for sand. I'm not interested in European beaches, so I cannot even tell you which ones in that area are sandy rather than rocky. However, the train runs all along the coast (though not out to the peninsulas, of course) as far as St-Raphael, so I think access to the beach will not be a logistical challenge. I'm not sure what the situation is with respect to free beaches vs. hotel-controlled beaches ($$$). Someone else will be able to help you there. You should expect any sandy beach accessible via public transportation and a short walk to be very crowded. You might Google for photos to get an idea.

The only sandy beach I saw was in Antibes, and it was tiny. I believe there's another that's farther from the historic center (and train station).

there are 2 similar threads running from the OP.

Answered your question on the other thread. Daytrips to the above (except St. Tropez) from Nice are easy.