( Dont miss - Musee de Louvre , Musee De Orsay ) , even one of these are gonna take you at least half a day . I spend 6 hours in Louvre but I managed to see what I wanted.
2) Else come during September when the whole Europe celebrates European Heritage Days ( have to check respective sites ), was there when Paris celebrated the Heritage Day so my visit to Notre Dame tower, Pantheon and La Conciergerie were FREE !

