3 weeks in Paris & day trips in 09/17

It doesn't sound as if you'll be spending as much time in Paris as you think you will.
While a half-day or day trip to Versailles or Fontainebleau is possible, the rest will take much more time - renting and dropping off cars, in particular. I'd recommend spending more time in Normandy - there's a whole lot of stuff to do there.

Woinparis explained the apartment situation - it's getting more complicated, and if you will be travelling around, it might not make sense to rent an apartment in Paris, where your belongings will be unprotected. Double door codes aren't as secure as people think they are, and if you aren't there for a few days, someone will likely take notice.

Instead, you could rent an apart'hotel while actually in Paris. Benefits of having a front desk staff, clean sheets when you come back to town and a kitchenette. There will also be a safe for your valuables and security cameras in the building.

Citadines and Adagio have apart'hotels in the type of neighborhoods you are looking for - less touristy but always convenient to transportation, markets, etc. Look in the 11, 12, 13 and 14 arrondissements - any Citadines in Montparnasse (14th) would be my choice.

If you want to explore the Loire valley, I'd look for a "gite" or one of the nice hotels in this area and spend some quality time:

http://www.chateaux-de-la-loire.fr/hotels_castles_loire.htm

I'd rent an apartment and plan to do a lot of day tripping. We once arrived late September but didn't have the apartment till Oct 1 and so spent 5 nights in Normandy FIRST and then headed to Paris. I would recommend that. Take an afternoon train to Bayeux on your first day and spend maybe 3 nights there for the beaches, a day trip to MSM (the Hotel Churchhill does a van tour) etc or rent a car and do it yourself, and then return to Paris. You could do the Loire as a day trip connecting with a mini van tour in Tours. And while in Paris there are dozens of great day trips.

I like Montmartre for longer stays; the area to the west of Sacre Coeur is charming and village like. See what Perfectly Paris might have for your dates; they actively manage their apartments and we have had great luck with them. We also really like the 14th near Montparnasse for extended stays but we stayed in a friend's apartment so I don't know local agencies for apartments. We also like the 17th but there are not many rentals there compared to more central areas or Montmartre.

The main thing about day trips is getting there and back (sorry, I guess that's pretty obvious) so if you map out your top priority day trips, that may indicate which of Paris's train stations you want to be near. Or at least which Metro lines take you to those stations with the least trouble. For example, trains to the Loire Valley (Amboise) leave from the Gare d'Austerlitz, whereas trains to Normandy (Bayeux) and to Giverny (Vernon) leave from Gare St. Lazare, and trains to Chartres leave from Gare Montparnasse.

Thanks for the suggestions. Sounds like starting in Normandy then working our way to Paris may be a more efficient use of our time. Bayeux sounds like a good base there.

In Paris, we'd still like an apartment rather than a hotel or hotel-apartment. We want to wake up & know we are in France rather than "anyplace at all". Any suggestions for reputable (legal) vacation rentals?

There are agencies which can legally list apartments. They are not breaking the law.
They do not vouch for the fact that the apartments they list are legal, and the majority of the apartments they list are not legal.
There are only 120 legal apartments which have been registered by the Mayor's Office.

Thanks for all the suggestions and insight. We have airline tickets purchased now & will be landing in the late morning on 9/1. Our thought is to head immediately to Bayeux and spend a couple days there & do the Normandy beaches (selectively) and then do a side-trip to Mt. St. Michel.

Now we're working out our itinerary for the return trip to Paris...thinking about doing some Loire chateaux and then maybe Fountainbleu before hitting Paris proper.

Lastly, thanks to the great suggestions for apartments in Paris, we are looking at the neighborhood west of Sacre Coeur.

Any thoughts or suggestions on the itinerary or things we should see / do that might not be on the typical tourist radar?

Since you will have a car think about doing Pierfonds and Vaux le Vicomte -- both of which are chateaux better accessed by car. Here was our trip to Vaux le Vicomte. We really loved it.
https://janettravels.wordpress.com/2010/09/20/vaux-le-vicomte-our-last-chateau/
Rouen is another place to visit on the way back to Paris -- very different from Paris.

I'm glad you are leaning to time outside Paris as well as an extended stay in the city itself. Day tripping is fine to a point - once transit times creep up to 2 hours each way, it does then become an inefficient use of your holiday time. Become smart with your packing and train (or driving) times, and move on to a new destination.

With 3 weeks...
Paris - 10N
Rouen - 2N (pick up car)
Bayeux - 2N
Mont St Michel - 1N
Loire Valley - 3N
Reims - 2N
CDG - 1N (depending on departure flight time)

I did the RS Paris and the Heart of France tour this fall. It covered some of the areas you are interested in. Have a look at their itinerary, you might get some ideas there. We stayed overnight in MSM, which was heavenly, then had a guided tour of the Abbey in the morning. The tour stayed in Amboise for 2 nights when seeing Chateau; I liked Amboise, saw da Vince's home Clos de Luce which was great. Also stayed 2 nights in Bayeux, book early for the tapestry and see their cathedral.

I love markets, I suggest looking online for market days where you are visiting. I have scarves from Amboise and many from Paris markets.

I suggest adding some guided tours or walks, they add so much history. After the tour I spent 2 weeks in Paris where I have been several times and I did a wine & cheese tasting at O Chateau, highly recommended. The area around O Chateau has the covered passages, I did not realize that when I visited and found one by accident, luckily! I had previously found an online brochure with the location and short history of the passages. Nice on a rainy day.

In Paris, Monoprix is a department store, some are large with groceries on a separate floor. Monop' is small with food often packaged in small amounts for picnics or quick meals. One Monop' in the 6th had a great wine area with a staff person to help you choose wine, how wonderful is that!

Enjoy Paris my favourite place in the world.

More later when I find more info on Paris.

You'll be in Paris for the 3rd weekend in September, for Les Journées du Patrimoine. Plan to stay in the city then and take advantage of public building openings - very very special.

Apartment: 68rivoli.fr

Overlooks the Hotel de Ville plaza; we're renting one of these in May. Fantastic price and equally fantastic layout/size/views! These are literally multi million dollar flats with a view across town hall plaza over to the Notre Dame de Paris cathedral.
Loire valley castles to definitely see: Chaumont Sur Loire. Azay Le Rideau. Chaumont has extensive beautiful gardens and multiple buildings. Azay is absolutely handsome!