WickedWesticle says:
British and Natural History are both really good.
I'm a sucker for stuff like the London Eye but it costs.
Tate Modern is ok. The Saatchi is awesome and it's not so big that you get bored.
Yeah the Natural History is cool...and the building itself is amazing if you take the time to have a look at that. The good thing about the museums is they're free.