For me a trip to London means a business meeting and journeys on the tube.  Time spent on damp windy stations or jammed in the heat of the rush hour.  Generally I have avoided it apart from doing the sites
- Madame Tussauds
- London Eye
- Buckingham palace
- Changing of the guard
- London shows
- Tower Bridge
- The Tower of London
and London Zoo
to name but a few, that I did as child.
However, if you care to spend some time walking around, looking up at the buildings rather than down at your feet or the other people, you can reap enormous befits in understanding the scale of the place and the closes association with the history.
One of my recent favourite sights of London has been the London Eye, built for the millennium, which form its location on the banks of the Thames, between Hungerford Bridge and Westminster Bridge, you can have a very good perspective on the key features of the city of London.  Beyond which the sprawling mass of a modern urban landscape washes over the land framing the river and the sites of, Big Ben, the Houses of Parliament, Buckingham Palace and Tower Bridge.  The furthest point up river to which we had sailed in the early Autumn of 2007.