Although it's not next door, we stayed in Dinard, which has the virtue of being a beach resort with good access to St. Malo. I found the immediate MSM area to be very touristy, but I'm sure the nice B&B's are slightly off that beaten, Alligator Farm (no kidding) track.
Except on the main roads, you should not expect to make fast progress with the car. You need a GPS (many threads to search for here) and a list of sights you might be interested in. Your stops would depend on where you're flying to, and where she works. I was sorry to miss the Paimpol steam train, and we missed some restaurants out of season, which won't be your problem. You'll have crowds. Parking at MSM was vastly modernized some six or so years ago, so it's less of a problem than it used to be.
We visited some local estate gardens, which were way off the beaten track. But it's getting late in the summer for many flowers. Here's a paste of the list of ideas from my 2012 itinerary: Dol-de-Bretagne menhir, M.S.M.,Dinan,St. Malo,Paimpol steam train/Beauport Abbey/moonlight walk Gardens: Coutances, La Ballue,Chateau de Caradeuc, Jardin Thabor, … de Beaumesnil, … de Bizy ...Vitré, Coutances or Cider Route
Further north: D-Day Beaches, Tapestry, Caen, resort towns/Lace workshop/war cemeteries/Honfleur
Gardens: Chateau de Brecy, ... de Canon, … de Harcourt, … de Vendeuvre,
Edit: I might have noted that Vitré is a possible substitute for touristy Dinan, if it's on your route already. It's not as good, but much less of a Disneyland. We didn't have time for Madame Sevigny's residence, just out of town. http://bretagne-vitre.com/le-chateau-des-rochers-sevigne/