Do you want to go in the water? I'm sort of doubtful that would be a comfortable thing to do in Brittany at that time of year, but I could certainly be wrong. Everyone likes St-Malo more than I did; the tourist load was a bit much for me. However, the town has legit sights to see, more than most coastal tourist spots, I guess. Dinard, Rennes and Dinan are all reasonable day-trips. Dol de Bretagne is close, but I haven't been there and don't know whether it's a good place to visit.
A different experience would be had in Deauville or Trouville. Perhaps no less touristy, but I think it would be more French tourists. Deauville, at least, is newer than St-Malo and wouldn't be as appealing if you're looking for pre-19th century architecture. And it's not any faster to reach than St-Malo, nor would the water be warmer (I assume). There's bus and/or rail service to Honfleur, Cabourg, Caen, and Le Havre.
There are some nice places on the west coast of France. They're likely to take a bit longer to reach from Paris. I liked Vannes and La Rochelle. La Rochelle puts you within day-tripping distance of Poitiers and two interesting islands, Ile de Re and Ile d'Oleron.