Things to do in London?

i m going to London with my older sister on friday and was wondering if anybody had any reccomendations of some nice cheap things to do while you re there (free to around £20 per person) any ideas would be gr8 thnx!
Answers

Anonymous

All the museums and art galleries are free; although some third party exhibitions require an entrance fee.

Trent

See Churchill's war rooms

sunshine_mel

There are literally thousands of things. What are you interested in?