Hi
we spent last Sunday afternoon at St. Jean de Beauregard. Website is here:
http://www.domsaintjeanbeauregard.com/wonderful garden and a nice little tour of the chateau. note this is not a huge chateau or grounds, don't expect too much, this is more of a peaceful afternoon in a pretty garden in the countryside. about 28 km from Paris, we got there by RER and then taxi (make sure you call ahead to book as they won't be waiting by the station); the taxi was about 10 euro each way. or if you have a car even better.
they have a festival/conference for gardeners in November and April.
this is the kind of thing you could do in an afternoon if you wanted to get out of paris for a bit, but i'd recommend spring, summer or early fall as you'll want to hit the garden's high seasons.
j