You could easily get to the Vendée in that time. It is a 3 hour drive from St Malo to the South Vendée. I recommend the area around Fontenay le Comte. Plenty to do, an easy drive to the beach and also the Puy du Fou and Futuroscope both of which are great for teens. There is a water park and adventure park (with high ropes) about 30 minutes drive.
There are loads of gites in the area, I recommend you get one with a heated pool.