Oh I thought of more!
Highgate Cemetary
Strange Things to do in London | UK Student News and Events
Ride around and sightsee from a double decker bus (free if you you have a travelcard)
King's Cross station (if you're a Harry Potter fan) - this is where they shot the scene where the Hogwarts Express picks up Harry on platform 9 3/4 ^_^
Covent Garden has street performers and artists everywhere.
Albert Bridge - (by South Kensington underground station) - you'll recognize it from
A Clockwork Orange