London Eye and Big Ben are the usual places to go. You can also visit the Tower of London, which has (optional) guided tours, and is really interesting to go to. The British Museum is a place you have to go, no matter what. You can visit Trafalgar Square, too, and see Nelson's Column. I do know that there is a Sherlock Holmes museum on Baker Street (of course) and a Jack the Ripper tour in Whitechapel, but as I've never been to either, I can't really recommend them, but it can't hurt to throw an idea or two out, right?