That's rather like asking what to do in NYC; personally, I say just grab an underground map and start going places.  See Trafalgar Square, Piccadilly, etc.  This is what I'd do, if you have time:
- See the changing of the guards.  
Here you go.  It's a tourist trap obviously, but it's fun to see.  
- Madame Tussads.  A lot of people seem really fascinated by it.  It doesn't interest me much.
- Walk around the Thames, see Big Ben, Parliament, Tower of London, Tower Bridge (not London Bridge), etc.  You could perhaps ride the London Eye.  Again, a tourist trap, but fun enough and the view is great.  
- Eat.  Plenty of restaurants around.  I'd hit some of the great Indian restaurants and some of your more ethnic places.  I had great food while in London, everything from Egyptian to Ethiopian.  
- See Westminster Abby.  Darwin is there, Newton, etc. 
That pretty much covers the touristy attractions.  I'd mostly suggest just walking around and having a good time; of course, that depends on where you are, but that's where the Underground map comes in.  
In summary: Go explore.