I have been to Paris a number of times, so this time I was looking for something different to do.
It was recommended to me to go to Les Catacombs de Paris, which is a large underground maze full of bones! It is accessible by Metro, and really quite fascinating. There are approximately 200 cemeteries worth of people down there, with their bones neatly stacked by date.
So if you are looking for something other than the usual Eiffel Tower and shopping, this is a great way to spend a couple of hours.
Here area a couple of links about it and the history:
http://www.carrieres.explographies.com/indexus.htm
http://www.catacombes-de-paris.fr/english.htm