The National History Museum .. Fascinating ... Of course, a trip on a boat down the Thames past all the historical buildings. don't try and do too much each day or you will just get fed up and it will be far too much to take in. Try and book for the big wheel thing .. Especially if it's a fine day, the view you will get will be stunning.
http://www.londontown.com/ is a good site .. Informative without being too stuffy.
Don't rush .. Again, take your time ... There is so much to see that you can't possibly take all of it in.
Ooh and an open topped bus is a must for the big attractions.
Enjoy!!!
Xxxx