I have been to London twice and I love it. I would recommend getting the London pass. It gives you free entry to a lot of the attractions and coupons for other things. You can also pay more and get a travel card with it that has unlimited tube/bus rides for the length of the card. I would also recommend going to Windsor; admission to the castle is included in the London pass and if you get the travel card, I believe the train is free as well. I didn't get the travel card last time but I remember having to show my pass on the train still. The castle is really pretty and Windsor is a cute town. As for as London, I have basically seen all of the major tourists sites and I liked them all. The London Eye is cool, and depending on when you go, you might be able to go inside Buckingham Palace. I think it's open for 6 weeks in August/September.
I went to Paris for a day the first time I was in London. I went on a Tuesday, so most (or all) of the museums were closed that day, so if you are there on a Tuesday, keep that in mind. The only things I saw there were the Eiffel Tower and Notre Dame, and I sprained my ankle the day before so I couldn't even walk up to the bell. We did go on a bus tour of the city so we could at least see most of the town, since I was in no condition to walk around.