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