Dunno about castles & abbeys etc, but stoneswise in the Carnac direction, don't miss Gavrinis (assuming they're running the boat trips when you're there)!
Also that way, there's also loads of ace stuff around Locmariaquer (like the Grand Menhir Brise, Mane Rutuel etc) & St Pierre Quiberon. Otherwise, around Carnac there's just bonkers amounts of good stuff. Generally The Megalithic European will give you the 'best' ones, otherwise try to get hold of Burl's Megalithic Brittany. (Or if you have time, use the Googlemaps on here!!) If you're trying to find stuff using Burl, I'd recommend getting the really good 'Blue' maps of the area....
Jane posted some blogs that might help, for example with putting together a 'shopping list' around Carnac. Unfortunately they were posted before TMA could host European sites, so they don't link to the site pages etc. All the sites are now on the relevant sections of TMA though.
Around Carnac with Spaceship Mark
http://www.themodernantiquarian.com/post/36663/weblog/
Some Carnac & Locmariaquer, Gavrinis etc
http://www.themodernantiquarian.com/post/36627/weblog/
Near Carnac & St Pierre Quiberon
http://www.themodernantiquarian.com/post/37318/weblog/
And some more near Carnac from me
http://www.themodernantiquarian.com/post/37386/weblog/
Away from Carnac, without looking up exactly where you are gonna be, I'd REALLY recommend trying to get to the stuff around St Just which I reckon is about the same distance as Carnac.
St Just http://www.themodernantiquarian.com/post/36904/weblog/
Depending on which way you're getting to Josselin, there's also the Roche aux Fees, which is quite near Rennes. If you're going that way - you could perhaps visit on the way there (or something!) - it IS spectacular!
Or if you're going from St Malo there's the amazing Menhir Champs-Dolent near Dol-de-Bretagne & the pretty Tresse alle couverte which are mentioned in one of J's blogs above.
North of where I think you'll be, there's some quite nice stuff around(ish) Mur-de-Bretagne, such as Liscuis which is again about the same distance from you as Carnac.
You might find this website useful http://megalithes-breton.fr/ though frustratingly you need to know the nearest town/village & I haven't found anywhere where it gives you an overview of an area.
This is quite useful too, though I don't think much stuff in it will be very close to you http://www.thecbj.com/megalith_guide.html - and you can D/L it as a pdf. (You may need maps to go with it, despite the fact he does try quite hard with his own!)
I believe Rennes cathedral's pretty cool & I think Vannes is quite nice. I seem to remember there are some nice castles in the biggest towns at the Brittany end of the Loire, but that might be a bit far.
I'm knackered now!
love
Moth
