Double Decker Bus Tour - You just HAVE to do that while you're in London!
I love Buckingham Palace and it was fun watching the changing of the guards.
I'd definitely spend the free afternoon shopping! Make sure you check out Harrods and Trafalgar Square.
I think Disney covers the most important sights in London. Since you only have so little time there, that's really all you can do. And it's all pre-arranged so you won't have to worry about getting around.
Also VERY cool that you get to go through the Eurotunnel!
The Hilton hotel is right "in the middle of the action" so that's a plus. Notre Dame, Eiffel Tower (worth going up to the top, IMO) - sounds good. Montmartre (as suggested) would be a nice choice for your free afternoon. Or you could spend some time strolling along the Champs d'Elisee. Also, you wanna spend some time shopping in Paris. The Louvre - worth checking out (if only from the outside for those - like me - who are not into art that much). We had breakfast at Centre Pompidou (right beside the Louvre) once which we enjoyed.
I've never been to Versailles myself but I've heard it's one of if not THE most beautiful castle in all of Europe. That's one place I wanna go sometime, too.
Finally, DLP: Is it worth staying the extra two days? DEFINITELY! Especially since you've flown all the way from the U.S. to Europe anyways and one week is awefully short...