In addition to the museums recommended: If you're flying into Heathrow, try and get yourself a right-hand window seat. Assuming a westerly wind component, you'll get great views of Central London on your approach weather permitting. That being said, from Central London, you'll also get good views of stuff on the approach to Heathrow coming in at about 3000 ft AGL.
London helicopter tours are also available. Although I can't make any specific recommendations on any organisations, I'm sure they all do a fine job
