I'm sorry, did someone say the M-word???
London's a great place, if you ignore the smog.

The museums that Minerva mentioned are great - get off the District or Circle line at South Kensington and make your way up to the Science Museum, Natural History Museum and the V&A - definitely worth a look. The British Museum is elsewhere (George Street station, I think)
Something that noone has yet mentioned is the Monument - it was built as a monument (duh

) to the Great Fire of London in 1666 and when you get up (if they haven't gone and closed it again) you get a great view of the city. Same with Tower Bridge. St. Paul's Cathedral is also something not to be missed, especially if you go up to the Whispering Gallery.
A personal recommendation is to go to St. Olave's Church on a lunchtime - they have free classical music recitals there, and if you're the least bit interested it's a great place to have your lunch whilst listening to some very good music. You'll find that a little north-west of the Tower of London, on Hart Street very near Fenchurch Street Station.