Loire Valley, Brittany, and Paris - ideas

sketterer,
10 full days, so you are not counting arrival and departure days as full days? Priority of Loire Valley? I would give it three nights, that means a half day after the train ride from Paris and 2 full days, leaving after the 3rd night in the Loire. Amboise is a popular base for the Loire. There are buses to get to some of the chateaux. (We always drive, so I don't know the details.) Chenonceaux is the one definite must-see chateau. Add another night here if you want. Rail travel from chateau to chateau is not an option. They are close enough together that you would want to drive, bike, or bus between them. And wine tasting is best by a tour. If you do have a car for this portion of your trip, be aware that the French are very strict when it comes to driving after drinking. Their alcohol limit is less than the US, and they do pay attention around winery areas.

The train from Amboise to Vannes is about 4 hours. A morning train gets you to Vannes about mid-day, so you could stay there 2 nights for a day and a half with the friend. Add another day here if you want. Vannes is a good place from which to explore Brittany, a unique part of France with its distinct culture and language. Or leave Vannes and stay somewhere in Brittany for a night. A car however would be the best way to explore Brittany. (Maybe the friend will take you on a drive through part of Brittany during your visit.)
Return to Paris for 2-3 nights before your return home.
So...Day 1 arrive, stay in Paris and get a taste of the city by wandering around or visiting one or two shorter time sights. Adjust to the time difference.Sleep Paris.
Day 2.... early train to Amboise, visit local Chateau in Amboise and/or Da Vinci's last home. Sleep Amboise
Day 3 and 4....take tours to two chateaux per day. Could be a troglodyte site in place of one chateau., or a wine-tasting tour. Research the area and pick two sights to see per day. Need to research bus options/schedules or local day tour options. Sleep Amboise two more nights
Day5 and 6....Early train to Vannes for 1 1/2 days here with friend Sleep Vannes
Day 7....extra day/night in Vannes or someplace in Brittany (Quimper, Carnac?) or return to Paris Sleep Vannes or Brittany or Paris
Day 8, 9 10 and 11....Arrive by train from Vannes or location in Brittany....3 1/2 days to see some of the sights and have at least one romantic dinner. (This could begin on day 7 if you don't stay that extra night in Vannes or Brittany.) Sleep Paris
Day 12....return home
This is predicated on your having 10 full days in France, as you said. If you are counting arrival and departure days, in those 10 days,you will have to cut out two days from the suggested list.
I wouldn't shortchange time in Paris. It is in a class by itself!
Have a great trip!

By Normandy, I take it you mean the DDay beaches. Normandy is a large area with lots more to see than Omaha Beach and Bayeux.
IMHO it is too much for a day trip. If you spend two nights there you could have one full day, which you would need just to visit a few of the DDay sights. And the all day tours are not cheap. Getting from Vannes to Bayeux or from Paris to Bayeux and back, and seeing a few things will make this detour a really long day. Best to save it for another trip when you have more time. Paris, Vannes and the Loire Valley will fill up your ten days very quickly! Especially as you are laid-back, you don't want to rush through things.
Bon Voyage!

A few spots in Bretagne that may be of interest, other than Vannes, would be the former German sub base Keroman at Lorient, built by the Todt Organization using slave labor. It's huge and pretty impressive. Made up of three blocks, a guided tour enters the largest one (K3). Lorient itself is kind of like Brest -- largely destroyed by the war and consequently not a beautiful place. About 45 minutes by car from Vannes.

The menhirs at Carnac are amazing and worth a visit, along with the associated museum. Have your friend call to find times for guided tours. About a half hour by car west of Vannes. These first two could be combined in a one-day trip, as you pass fairly close to Carnac on the way to Lorient.

Pointe de Raz is quite a scenic spot. If you visit on a windy, stormy day you may find it hard to stand -- the wind can blow so hard. About 2 hours by car west-northwest of Vannes, with opportunities to stop at Quimper and Douramanez on the way there or back.

Sounds like a lovely trip. We did three nights in the Loire Valley without a car a few years ago, and what we found easiest was staying in Tours (which I also thought was a lovely town). You can get the high speed (TGV) train from Paris with just one short change at St-Pierre-des-Corps that takes you directly to Tours. From there you can get trains to both Chenonceau and Amboise very easily. For the other chateaux, we joined a day trip in a van with a small group which took us to Azay-le-Rideau, Villandry (gardens only), Cheverny, and Chambord. Once we were at each site we visited independently- the van was just an easy way for us all to get around. That worked well for us and three nights was a good amount of time. Good luck!

Just a comment on the plan to see "castles" on this trip. The chateaux in the Loire Valley are huge country homes and not what most people would think of as "castles." Chinon is probably more "castle-like" and I imagine other readers may know of other older, more fortress-like structures in that region.

Some random thoughts from our trips to France:
1. La Roseraie is a lovely hotel in Chenonceux - you can walk from there to the chateau. We also enjoyed Leonardo's home in that area. We did not visit any of the other chateaux.

2. parts of Brittany near Vannes: places near Vannes include the huge rows of megalithic stones at Carnac, Auray, Rochefort-en-Terre. Suscinio castle is also in that area - it looks like a real castle, nothing like the chateaux / flamboyant mansions of the Loire.
3. North coast of Brittany: St. Malo is very interesting, Dinan is a pretty medieval town. Mont Saint Michel is about 40 miles east of St. Malo. It is in Normandy.

4. The north coast of Brittany is more dramatic than the south coast of Brittany; however, not as dramatic as the Pacific coast or the New England coast.

With ten days, including Paris, you only have time for small sample of northwest France. The notes above are IMHO the highlights.

Just a comment on the plan to see "castles" on this trip. The chateaux in the Loire Valley are huge country homes and not what most people would think of as "castles." Chinon is probably more "castle-like" and I imagine other readers may know of other older, more fortress-like structures in that region.

Château de Fougères would qualify, I would think -- one of the largest defensive medieval castles in Europe.

Everything that Judy said.

50th birthday. Loire. Chateaux. No Car.

To me, that's easy. Bicycle - or e-bike! See chateaux, small towns, markets, countryside. Maybe a winery around Vouvray? Place Plumereau in Tours? Meet locals ... the bikes and picnics along the way spark conversations.

A great starting place to look is https://www.freewheelingfrance.com/. This site collects LOTS of information for getting around on your own as well as a number of operators who put together self-guided trips.

On the freewheeling website, you can zero in on the Loire and find four or five operators. You'll find packages from as short as three days (2 nights) to 7 or 8 days (6 or 7 nights). Many will likely be willing to adapt their itineraries to satisfy your needs. Two years ago, we cycled one week in the Dordogne and another week in the Loire (starting in Blois, then Amboise for two nights, Tours, Azay le Rideau, Chinon and Saumur, and visiting 7 chateaux and the Abbaye Fontevraud, plus walking or biking past about four more chateaux). The outfit we used, Velo Voyageur allowed us to modify their packages to suit our wishes. You can find them on the Freewheeling France website. Also see our TR. https://community.ricksteves.com/travel-forum/trip-reports/fabulous-tour-de-france-2-weeks-on-foot-and-by-train-2-weeks-by-e-bike.

If you’re wondering about your baggage, they arrange all luggage transfers, so you just need to be packed by 9 am and get on your bikes. The bags are at your next hotel when you arrive mid to late afternoon, after you’ve ridden, taken in a sight or two, had your lunch or a picnic and are ready to take a shower before visiting your new town and look for dinner.

The Loire is full of well marked cycle routes, so if you would prefer to handle the logistics yourselves, that’s doable, but I would choose the let an outfitter do that. The cost was really quite reasonable (self-guided is MUCH cheaper than guided) and they did a pretty darn good job selecting hotels that were generally charming and nice - both in terms of quality and location. On each of our two one-week tours (Dordogne and Loire), we quite enjoyed 6 out of 7 hotels. A better batting average than I might have had than if I had struggled to assemble it all on my own.