I only had about 10 hours in this city so my experience is limited. However, I was able to sit some popular hot spots.
I was able to visit the Buckingham Palace. It is hard to believe that someone actually spends their days in such a luxurious place. The tour was great. We were able to tour a good amount of rooms while still learning a great deal about the palace. Every other day you can watch the famous "Changing of the Guard". This is the top of my "Must See" list for London.
Knowing how to use the bus or train station in London is essential for maximizing your time in this major city. I unfortunately did not educate myself on how to use such systems (I grew up in a small town) so I had to walk everywhere. Not recommended if you have limited time.