This is an really good beach not that far away from Plan (about 30 - 45 mins drive depending on traffic.). It has soft sand, a great view and easy access.
NOTE: It's a shorter drive from Plan if you take the hill road to the infamous la foux roundabout and follow the signs to la Croix Valmer from there. A little longer if you drive into Ste Maxime and take the coast road to la Foux. Avoid St Tropez Market day on tuesday if you can!)
The beach itself is just past the pretty floral village of La Croix Valmer and is off a roundabout on the left hand side. There is plenty of parking (free out of season) and the beach is approached through a path lined by shops and restaurants. If you fancy a picnic there is a Spar at the other side of the roundabout.
The locals have cleverly constructed a jetty which you can walk down and where ferries to and from the various islands in the locality stop in summer season. There is a ticket booth with timetables etc on the walk to the car park.
This beach itself was one of the landing points for the Allied Invasion of Southern France which began here on the 15th of August 2015. There are a number of plaques marking this event including the one below.
Incidentally, ff you are ever in this region on 15th August there is a large military vehicle parade which passes through Ste Maxime and Plan de la Tour.
The view from the beach back to land is stunning and the many restaurants and bars offer plenty of choices.
There are a number of sailing schools on the beach where all sorts of boats can be hired by the hour. There are also public toilets and beach restaurants.
The beach itself stretches all the way to Calavaire and is at least 5K long on one side. On the other side there is a coastal path which goes back towards St Tropez via Gigaro beach.
When I visited in April there were magnificent flowers growing everywhere on the sand.
Finally, if you are a feeling bit energetic you could do worse than walk/Jog down the beach towards Calavaire and about 2 Km along the way there is a cute cocktail bar selling excellent Mohitos!