Just to update you since you said that museums are high on your list:

While in Paris all of last week all of the museums and monuments were closed due to a strike. Each morning the staff would take a vote to determine if they were going to work or not and each day they didn't. Many people who had purchased the carte for the museums were very upset (and rightfully so) that they could not get a penny of their $$ back. You may want to wait and make sure that the museums are open before purchasing the card. I know it means that you have to stand in line a bit more, but personally I think it's better than being out ~$50. There is a website that posts whether or not the places are open. However; having said that, Paris is still amazingly beautiful!