I've a Navigo for zones, but I'm not sure if it's valid to go for Chantilly Castle.
My credits are valid from Gare de Lyon to Chantilly Chateau?
Since the Navigo Semaine package is valid throughout the Île-de-France zones 1-5, it could not be used for a direct trip to Château de Chantilly, which is located outside Zone 5.
SNCF shows it as a 50-minute journey from Paris Nord to Chantilly-Gouvieux for 8,70 €; the castle is about a 15-minute walk from the station.
Rome2Rio is also a useful tool to explore options and alternatives, and a map of your route. And @pnuts has thoughtfully provided this map of all your transport options within all of the Île-de-France zones.