So hard to even start!
Houses of Parliament
The London Eye
The Tate Britain/Modern
The South Bank
Tower Bridge
The British Museum
The National Gallery
The National Portrait Gallery
Hyde Park and the Diana Memorial
And these are just the first ten that come to mind. Does the tourist like parks? Does the tourist like good food? Does the tourist like history? Does the tourist like politics? Does the tourist like film/theatre?
There's no end.