We always (rather boring I know) lunch at La Duchesse Anne. If the taxi drops you at the customary 'Centre Ville', go through the archway, turn left and the Duchesse Anne is there on your left. But there are loads of places to eat in the old walled town.
I would recommend booking accom before you go though.
People always say, although I've never done it, that you should clear customs at Dinard and then fly on the extra five minutes to Dinan. May be easier to get accom there.... but I don't know
