If you’re in the mood for bucolic country hikes, medieval fortress ruins, and secluded château visits less than an hour from Paris, head to the Natural Regional Park de la Haute Vallée de Chevreuse.
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Vallée de la Chevreuse, Château Breteuil, Château de la Madeleine
Just under an hour southwest of Paris by train, this protected regional park is rarely visited by foreign tourists (which is probably how the French like it), but has many excellent sites easily accessible by RER and the seasonal Baladobus, a shuttle that circulates every Sunday in April through October. Starting from the RER B station Saint-Rémy-lès-Chevreuse, the Baladobus (€4 for a daypass) stops at several historic hiking trails leading to many unforgettable sites such as the 11th-century ruins of the Château de la Madeleine (with panoramic views over the valley and a visitors’ center), France’s oldest riverside mills, a house made of iron, charming villages hidden in wooded valleys, vestiges of the 13th-century Port Royal Abbey (now a museum), and the magnificent Château de Breteuil with its formal French gardens.
Note that you can also bring your bike and cycle many of these trails. Pack water and snacks (not much is open in the smaller villages on Sunday), and have an actual map in case your GPS reception disappears in the forest. All of the information is online in English with full descriptions of each trail, hours and prices for each site (you can also download the PDF leaflet).
This is going on the to-do list. A few weeks ago we finally got around to following your recommendation to see the Albert Kahn garden and it was just wonderful and completely new to us. (We have lived in Paris with a few interruptions for nearly twenty years so we have no excuse except that there is too much to do here.) Do keep the excursion ideas coming!