If you can only make one stop in northern Brittany, we highly recommend Tréguier. This medieval town is well preserved yet not at all touristy. There are a few tourists, of course, but nothing like in Honfleur, for example.
Parts of the cathedral date to the 11th century, but most of it is from the 15th and 16th, still plenty old. The houses are mostly stone or half-timbered. Most have brightly painted shutters, and many have flower boxes too. It can be gray and gloomy here, and the colors help a lot.
The marina is small with mostly local boats. The visiting British sailors are friendly, though, and we heard a lot of English spoken along the waterfront where the restaurants and bars seem reasonably priced. Shirlee enjoyed her birthday here at an authentic Breton crêperie.