Assuming you haven't been before? First of all see if you can get hold of a copy of Time Out to research if there is anything special you'd like to see whilst you're in town, and plan your itinerary around it.
Otherwise I would say the River Thames is a must-see. Get to Westminster & take a boat to Greenwich. You'll see lots of great things, like Houses of Parliament, Waterloo Bridge & South Bank, Tower Bridge, Tate Modern. Once in Greenwich you could stroll around (it's got a very interesting Maritime Museum), or you could head back to town and hang out in Soho, which is still a fascinating area of London. I'd definitely make a stop at one of the great parks like Hyde Park, Regents Park or St James's Park (the latter 2 have the best cafes). Then of course there are the art galleries, museums, gigs and theatre. Depends what you're into, really--I like art and food when I am travelling, so I would say the ICA is always worth a visit, then hop on the tube to Liverpool Street and visit Brick Lane for a great curry. But that's just me!
Oh and if you have time, there are some interesting markets you could visit: everyone's heard of Portobello and Camden, but you could also check out Brixton, or Southall, for something a bit different.
Have a lovely time whatever you decide to do. I've lived here most of my life and still love London.