There's not as much down time in London, but we really enjoyed the British Museum. (It's only about a 10 minute walk from our hotel, and we spent about two hours there, seeing only about 1/5 of the museum.) We also did the London Eye on some of our down time, but we weren't overly enthusiastic about it. (If you go, go in the late afternoon when the sun has started to go down or in the evening. The bubbles are poorly airconditioned, and when the sun is shining brightly on them, it's very hot -- uncomfortably so.) But many families thought the London Eye was magnificent, so, go figure. If I had a choice, I would have gone back to Westminster Abbey and spent more time there in London, I think.
Your plans for Paris sound wonderful. The evening Seine cruise would allow you to see the Eiffel Tower in the dark. Going to the Moulon Rouge would put you over in the Sacre Coure area. Many of the people who went there in our group said that it was the highlight of their trip (Sacre Coure).
You'll have a great time. We're still amazed at how much we did. Austin (my son) is eager to work on his trip report today again. Can't wait to see what he says.
Julie