Now, this is one of the most best kept secrets of the Parisians : it's one of their favorite museums and they have managed to (almost) keep it a secret! Not only is the building itself (a beautiful hôtel particulier built in 1876 not far from the Arc de Triomphe by a banker, Edouard François André) exceptional, with a permanent exhibition of his incredible art collection (most of it was acquired by his wife), it also often hosts interesting and fashionable temporary exhibitions. Not to mention the very cosy restaurant where elegant retired ladies (but not only) like to meet!
Repeat visitors to Paris who have never been there before, you just HAVE to go there some time.
The visit takes about one hour, add another hour if there is a temporary exhibition you want to see as well (it's possible to buy separate tickets) and add an extra hour if you stay for lunch!