Albert Kahn was a banker who, near the end of the 19th century, began building what turned out to be a fantastic garden at his home in Boulogne, on the western side of Paris. He also amassed a large number of photographs during his extensive travels. For a very modest entry fee both the photographs and gardens are available to the public, and getting there is quite easy. From central Paris, take Métro line 10 to the end at Boulogne/Pont de Saint-Cloud. The entrance to the museum and gardens is just off the place. http://albert-kahn.hauts-de-seine.fr/english/
The museum is undergoing an expansion, and some of the garden areas are being refurbished, so I was unable to see everything I wanted. However, it was well worth the visit.
This would be a great place to see masses of color in both early summer or in the fall, but, even during the heat of the late summer, it was a delightful place to stroll. Other than a few visitors, and a couple of artists with easels, the gardens were quite empty.