Duck tours are a good way to see the city sights...its a kind of land rover x boat which drives round the streets then floats down the thames. See the link below:
http://www.theyellowduckmarine.co.uk/

Hyde Park Winter Wonderland might be worth a visit, christmas market stalls, rides and an out ice rink.

http://www.hydeparkwinterwonderland.com/

EDIT:
This might be useful depending what you are planning see http://www.londonpass.com/index.asp?...Fcts4wodA2ikqg