We are primarily interested in the cathedrals and the old town areas. Which would the forum suggest is the most interesting ?
Rennes! Rennes is a very beautiful city in Britany where you have a beautiful cathedral and its region has a lot of wonderful natural sites but also so many churches, dolmens and menhirs and small fishermen villages.
A few hundred Kms away is also Quimper and its region.
In the Region, Nantes, La Rochelle, Cognac, Saintes are interesting places to see, you need a car, otherwise travel is very long in these places.
Try oysters.
In Brittany in general, including Rennes and around within a short distance, there are many beautiful towns and churches to visit. For example you could consider stopping in Dol-de-Bretagne and its cathedral, either on your way from Rennes to Saint Malo or to Mont Saint Michel. Dinan or Fougeres also have 2 beautiful churches besides their medieval town.
About Rennes you can find some information about how to come, events, etc. on http://visite.bretagne.free.fr/index.php/en/cities/rennes.html