Chantilly, Senlis, Pierrefonds, Compiegne, reasonable itinerary for 3 days 4 nights.

For Château de Compiègne, Château de Chantilly and Mémorial de l’Armistice you don’t have to buy online according their websites. Only Château de Pierrefonds strongly advices it buying online (conseillons fortement d'acheter vos billets en ligne).
https://chateaudecompiegne.fr/en/individuals-or-families
https://chateaudechantilly.fr/en/tickets-prices/
https://armistice-museum.com/preparing-your-visit/hours-and-prices/
https://www.chateau-pierrefonds.fr/en/Prepare-for-your-visit/Practical-information2

I have visited all the mentioned places except the stables and the hamlet of Chantilly. Even being a 19th century creation I like Château de Pierrefonds too, but the inside is a bit empty, there is a little exhibiton about architect Viollet-le-Duc. The castle is one of his creations as well as that famous spire of the Notre-Dame in Paris lost during the fire of 2019. Pierrefonds itself is very nice, so certainly worth a visit.

Also Compiègne is a pleasant place and as the palace there is much overlooked there were just a handful of visitors 5 years ago. It houses a little car museum too. Despite the wagon where the Armistice was signed isn’t the original one is Mémorial de l’Armistice worthwhile to visit too.

There are many charming villages / hamlets (depending your expectations, but I liked them) to discover like Saint-Jean-aux-Bois, Saint-Crépin-aux-Bois, Morienval, Longpont and so on. Didn’t visit it but think the historic centre of Crépy-en-Valois is charming too and in Villers-Côterets is a little but nice museum about Alexandre Dumas. Ermenonville isn’t bad either.

For planning I use the yellow cover Michelin Departemental maps, in this case number 305. Worthwhile is following the green lined roads.

There are a lot of walking trails, best is to contact the local TI’s. As I can read everything in French I can't remember English explanations in the museums, so can't help you with this.

Of the 4 listed here, I am only familiar with Compiegne and Chantilly, went once to Compiegne and a couple of times to Chantllly,
always as a walk-in. Both are deeply historical sites. The other two I have not yet visited.

The Chateau de Chantilly belonged to one France's victorious military commanders aside from being a member of the Bourbon ruling dynasty., known as the Great Condé, the victor of the decisive battle against the Spanish. There is a painting of that battle, Rocroi, in the Battle Gallerie in the Chateau de Versailles.

Compiegne was the site of the Armistice signed on 11.11. 1918. When you go through the railway car (replica) depicting that event, notice that although the US actively was engaged in almost non-stop combat against Germans from July 1918 to Nov 11, no US representative, either military or civilian, is seated representing the belligerents at the table in the railway car.

part 2 here...

Aside from the events of 1918 and 1940, when the French lost and had to submit to Hitler's demand that the cessation of hostilities be signed in the same place as was done 1918, (there's documentary film of the 1940 event), see the Museum on WW1 on the premises.

From the site of the railway car, the Deportation Memorial is within walking distance. This entire area of Compiegne is a poignant site.

In town is the Chateau, well worth seeing if you're deeply into Napoleonic history....captivating and fascinating. I took a guided tour of the Chateau given only in French, didn't understand too much though.

I am intimately familiar with all of the towns you mentioned (Senlis, Chantilly, Compiègne, Pierrfefonds, etc.) as well as that whole area in general (Crépy-en-Valois, Villers-Cotterêts, Morienval, Vallée de l'Automne, etc.) and I see that Wil has more or less given you most of the information I was going to provide you with. Just the other day I was corresponding with someone on another travel forum who was seeking my advice for that specific area so I'll copy and paste the info I provided them with which will likely be useful to you:

Ok, I've got some ideas for the Chantilly / Compiègne part of your stay. If you haven't got it already then I recommend getting Michelin map # 305 Oise, Paris, Val-d'Oise.

You mentioned rail trails converted to bike trails and there is one of 13km in length connecting Compiègne and Pierrefonds:

http://picardie.media.tourinsoft.eu/upload/CR7-FR.pdf

I'm sure you know all about Pierrefonds and its château but not far off is one of France's Plus Beaux Villages called Saint-Jean-aux-Bois. It's tiny but pretty with a well preserved collegiale church and I believe one of the two restaurants in the village has a Michelin star:

https://www.villagesetpatrimoine.fr/saint-jean-aux-bois/

https://sebastien-tantot.com/fr/

Not far from St-Jean is the Gallo-Roman amphitheater called Champlieu:

https://www.oise.fr/actions/culture-et-patrimoine/archeologie/sanctuaire-de-champlieu

Across from the amphitheater you'll notice the little plaque/memorial commemorating the fact that the field you are looking at was where tanks were first tested before entering service in WWI.

If you want to do a pretty drive on a country road then nearby start in Bethisy-Saint-Pierre (which has a nice château and park) and take the D123 east toi Villers-Cotterêts. Along the way the roure name changes to the D32 then the D231. You'll see a few châteaux, including the Château de Vez and the abbey ruin of the Abbaye de Lieu-Restauré:

https://www.bethisysaintpierre.fr/?page_id=432

https://www.donjondevez.com/en/

https://abbayedelieurestaure.com/

Most villages in this area have churches designated as historic monuments. One such village is the pretty village of Morienval with its collegiale church and garden, which are rarely open to the public but you can always try:

https://www.valois-tourisme.com/Visiter/Cote-jardin/vallee-automne/122

https://www.francethisway.com/places/a/morienval-oise.php

Nearby is the pretty little town town of Crépy-en-Valois with some nice medieval streets (cobbled and maybe not good for a mobility scooter). It's 12th century château is now the archery museum (town history/tradition is tied to archery):

https://www.musee-archerie-valois.fr/2021/09/01/le-chateau-de-crepy-une-architecture-remarquable/

https://www.valois-tourisme.com/

Closer to Chantilly are a couple of lovely abbeys you can visit and they are the Abbaye de Royaumont and the Abbaye de Chaalis. I've visited the exterior of Royaumont and it was pleasant but I visited both the exterior and interior of Chaalis and the interior is quite interesting with a lot of well decorated rooms, period décor and furniture (not sure it's handicap accessible, though). Worth a visit. Just down the road from Chaalis is the pretty little village Ermenonville and its Jardin Jean-Jacques Rousseau:

https://www.royaumont.com/

https://www.domainedechaalis.fr/

https://www.francethisway.com/places/ermenonville.php

https://www.oise.fr/actions/culture-et-patrimoine/culture/parc-jean-jacques-rousseau

Since there are a maximum amount of characters I'm allowed to post on this forum I'll have to continue here with what I was saying in my last post:

If you start from Senlis heading east on the D330 it's a pleasant drive passing through some pleasant villages and you can make a few photo op detours along the way for the Château de Mont-l'Evêque, the Abbaye de la Victoire and the Château de Versigny and combine that with a visit to Chaalis/Ermenonville:

https://destination.parc-oise-paysdefrance.fr/histoire-patrimoine/les-chateaux/chateau-de-mont-leveque/

https://www.visitez-versigny.fr/

https://www.chantilly-senlis-tourisme.com/patrimoine/ruines-de-labbaye-de-la-victoire/

Note the menhir on the roadside at the entrance to the village of Borest:

https://fr.wikipedia.org/wiki/Fichier:Borest_(60),_menhir_dit_la_Queue_de_Gargantua.jpg

I should add that with 3-4 days in that area I could probably give you even more info about places to visit in the environs of where you mentioned but I think I've mentioned enough for now and if you want further suggestions then say so and I have other ideas.

Reading your knowledgeable posts I immediately got the feeling remembering you as French Mystique from Lonely Planet’s Thorn Tree forum years ago. And indeed you are. Good to learn there is so much more to see north and north-east Paris, thank you for the info.

Hardly known is that there is a wealth of charming places around Paris. For all those like to travel off the beaten path and not looking for the world famous places but want to enjoy the charm you don't have to look further to my opinion, so actually you don't have to go to the other side of the country for this. I enjoyed too exploring the regional nature parcs of the Vexin and the Haute Vallée de Chevreuse.

If you've seen the movie King of hearts, it was filmed in Senlis.

Thanks for the nice words Wil. I enjoyed the Thorn Tree and was sad to see it go. Happy to know you're here and I'd imagine some other Thorn Tree members are here too.

Thank you for your recommendations and replies. A lot to look through! I'm still sorting the best place to stay as a homebase. There is so much in this region to explore. I'm hoping it will feel a bit off the beaten path. Again, many thanks.

"I'm hoping it will feel a bit off the beaten path."

It will feel a lot off the beaten path. Have fun. :)

Re: "the beaten path"

When you are exploring Compiegne and seeing the historic sites pertaining to WW1, etc , the chateau, and the city's centre ville, it will seem you are indeed off "the beaten path."