I would recommend Bourg St Maurice.
http://www.lesarcs.com/?lang=en
No good if you are insisting on a beach.
But if you like hill walking, kayaking, white water rafting, swimming in a 50ft outdoor pool, parapenting, mountain biking, road biking (good cycle paths), canyoning, climbing on and on its a fantastic spot.
The weather is great.
The nearest airports are Geneva, Grenoble or Lyon. All 2 1/2 hr drive through the Alps.
The cheapest way to get there is rent a car. there are buses and trains but they are pricey and if its more than 1 person, car hire is better value.
High season is in winter and there is tons of accommadation, so owners are more open to renting for 4 day stretches. I would stay in the town , not in the ski stations as they are pretty dead in the summer.
If you are planning activities you probably need to reserve them in advance.
The "nage in eau vivre" or swimming in white water with a boogie board and flippers is said to be a blast.