I was there in 2005. As others have said, worthy sights include:
Tower of London, the Ferris wheel
LondonEye,
London Aquarium, Picadilly Circle, Buckingham' Palace area,
Museum of London, the parc behind Downing street, Kensington Gardens, Westminster Abbey, accross the street from "Big Ben" and the British Parliament buildings, London zoo. There is so much, much more to see!! Make sure to bring lots of films/memory cards!!!
Don't be affraid to walk around, out of the usual tourist areas!
By the way, the aquarium is accross the bridge from Big Ben. When you are near the bridge, going "HOOO.... HAAAA...." at Big Ben, look accross the river. You will see the Ferris wheel. The aquarium is close to it. Just cross the bridge, turn to your left. Follow the river and the entrance to the aquarium will be on your right hand side. It is fabulous!! You will need a rubber band to keep your jaw shut.
When we were there, they were building octopus tanks connected with a glass tube. Should be very interesting to see. Also, do not miss the petting area with the manta rays. Very neat and they are very friendly!
Also, if you like flee market shopping, you have to go to Camden Town, Norht of London. The tube (Northern Line) will bring you right up there.... You can spend hours in there! When coming out of the flee market, turn right instead of turning left (to return to the tube) and there is another area, on the other side of the small river. They have more small shops, stands... Its really a neat area!! I'm looking forward returning there soon!
Have a safe trip, relax and enjoy yourself!!
HTH