Brittany 5 days

Brittany is huge. Many travelers choose at least two bases. We really liked Locronan, a small and thoroughly gorgeous town by Quimper. People often base in Quimper. The Dinan area has a lot to see. In addition to Dinan, there is Dinard and Saint Malo. Quimper and Dinan are a good 2 hours apart by car.

We used Dinan as our base last November and loved it. I recommend doing what we did: TGV from Paris to Rennes, rent car there, then drive the hour to Dinan. We did day trips to St. Malo, Mont St. Michel, and to see our dear relatives in Morlaix.

Agree with jules m, Brittany is huge. We based in Quimper for 1 week. TGV from Paris right into Quimper then rented a car for easy day trips from house we rented. Benodet, Pont L'Abbe, Concarneau, Audierne, Point Du Raz, Point Du Pen Hir. Had a great time trying out all kinds of creperies finding the best crepe and eating mussels, Moules Frites. We didn't care for the cider that they specialized in Brittany. Didn't think we could pack in Mont St. Michel into that trip.

I do not recommend driving from Paris. It is a long, boring drive. The fast train to Rennes is really fast, 1.5 hours, and you can rent a car right at the station. You can do the same from St Malo, 2:15 from Paris. The train ticket will cost about as much as gas and tolls from Paris to Rennes/St Malo.

We did 5 nights without a car a few years ago. 4 in an apartment in St. Malo which we much prefer to Dinan, but to each his own.

https://janettravels.wordpress.com/2020/11/24/all-the-light-we-can-see-st-malo/We spent one night in Auray and hired a guide with a car to take us to the neolithic site in the area -- it was wonderful. It would be very easy to do on your own with a car and we would have in the days we drove in France. https://janettravels.wordpress.com/2020/11/29/yes-ed-more-rocks-the-neolithic-alignments-at-carnac/

Well train tickets for 3 people was almost $200. My car rental is like 300 for the whole week. So not sure how that's cheaper. Plus we can stop in Loire and other places if you want.

With cars you have very expensive gas and tolls -- but for a family group in a fairly rural area it really is the best way to go -- you can visit towns that are hard to reach and the various neolithic sites. We would have done our trip by car if we still drove in Europe. My husband can't and I don't want to. And driving also makes MSM easy to do.

I know the two travelers who created the TR below personally, in fact I've slept with one.
Maybe something in this loooong TR could be of some use?
I am done. The end.
https://community.ricksteves.com/travel-forum/trip-reports/l-appel-de-mer-a-southern-brittany-tr-inc-bit-of-p

Despite what others might have mentioned about "very expensive" tolls in Bretagne, I'm pretty sure there aren't any. Bretagne and the Cotentin Peninsula in Normandie are two areas of France where there aren't toll roads. You'll hit tolls getting there, but once there, no tolls.

As for your plan, if you're interested in coastlines and seafood, Dinan is not really the best place for those. It's a fine town, no doubt, but Vannes or Roscoff might be better choices. Paimpol or Perros Guirec might also bear consideration. I'm more familiar with the seaside towns of Normandie, but from the time I've spent in Bretagne, those would be my suggestions for you to consider.

Nantes doesn't get much visitation from the US, but we enjoyed our time there. The Jules Verne museum is fascinating, Les Machines de l'Île and the adjacent shipbuilding museum are as well. The Château des ducs de Bretagne shouldn't be missed, and the Memorial to the Abolition of Slavery presents a history of the African slave trade that most Americans aren't fully aware of, as Nantes reminds itself of its own contributions to the enslavement of so many.

Nantes is certainly a somewhat gritty place with evident economic challenges, but it's also very genuine with a great deal to offer. I think more people should give it a try. As a full disclosure, I don't care for Breton (or Normandaise) cuisine, and Nantes offers a variety of foods that provide relief from galettes and buckwheat.

Dinan or Cancale are too far away to be a good base for whole Brittany
A good idea as already indicated would be to take the tgv to Rennes and rent a car to go to Mont St Michel then to Brittany and do a road trip in several stages (no there are no tolls)
The most interesting things to see in my opinion are in Northern Brittany
1)the region of Dinan Dinard St Malo Cancale
then 2) fort la latte, cap fréhel, perros guirrec, Ploumanac’h,
then a short tour towards Huelgoat to go down towards 3) the Crozon peninsula and Quimper,
on the road to Vannes les rias towards quimperlé (I am chauvinist I live there..merrien brigneau, doelan, rosbras)
and finally 4 or 5) le golfe Morbihan (Ile aux Moines, Pointe d’Ardon, the coastal road to Sarzeau) and the Carnac menhirs)
then return to Rennes
It could be done in 5 days but it’s speed

To second what others have said: Brittany is huge. A Train to Rennes would certainly be more pleasant. We took the train to Rouen, booked early for a grand total of €18 for two people. Picked up a car in Dinan, our main base, using vrbo. We loved our time there and would like to return but a lot of this was because of the specific apartment where we stayed, right on the river that divides Dinan from the next town. Two restaurants and a bakery directly across the river. Cannot remember the name of the rental but the owners name is Jeremy. Perhaps you could use Dinan or St. Malo is a base for the Eastern part of your trip. Locronan or Quimper sound like good locations for the more western part of your trip. I'm sure you'll have a wonderful time whatever method of transportation you choose.

As for your plan, if you're interested in coastlines and seafood, Dinan is not really the best place for those. It's a fine town, no doubt, but Vannes or Roscoff might be better choices.

We stayed in Vannes several years ago. Delightful medieval core but it isn't really on the coastline. From the base of the harbor (ringed by a few nice restaurants) it is more than a mile on foot to a larger estuary. The actual coastline, the Gulf of Morbihan, is more than 3 miles as the crow flies but since you can't fly it's much more than that since you need to circumnavigate the estuary. So if the OP is looking for coastlines Vannes probably isn't the best fit.

drink local booze, eat seafood

Brittany is on my to-do list. In addition to cider and lambig, do they have any beers specific to the region or local beer brewers of note?

The 3 most important breweries are Britt, Lancelot and Coreff
I like the brasseries la dilettante, skumenn, brasserie du bout du monde
La tête dur brewery, Bosco brewery, Baril brewery, Morgat
There are many small breweries in Brittany, let yourself be tempted by those in the area where you are
you can find the beers quite easily in markets, in regional product stores or even in supermarkets and of course innthe breweries... for cider Don't hesitate to taste "les bouteilles à l'amer" à clohars carnoët, especially the special vintages...they are really good and the owners are very friendly.

there are aslo somme good whisky and Gin in Brittany
you can see
distillerie des Menhir (eddu is quite good)
distillerie de la mine d'or
distillerie warenghem
and you must taste the Breizh cola (our breton coca cola ;)

Cancale has outstanding oysters. When I visited it was nothing special other than nice venues for a casual dinner with very reasonably priced oysters and beer and a view of the bay. The tide is interesting, boats float at anchor part of the day and lie on their sides on the bottom for the rest. It might be something to plan a meal, or two while staying nearby (MSM or St. Malo?).

I loved Dinan for the historical center. I understand they had a fire in the center since I've been there but never heard what was destroyed.

I think a trip south to Carnac would be worth your time. Stay in Auray and visit both Carnac and Cairn Givrinis (French only boat tour but worth it even if you don't speak much French - think Newgrange, in Ireland without crowds)

Edit: Vannes is also nice. I stayed there. It's much larger than Auray so the smaller town would be my first choice.