A must see in London

A must see in London?

What is a must see in London, what to do that just has to be done while you're here?

Other Answers:

I would advise you go to the Science Museum, because it is free and world class in terms of the exhibitions it has on display. Really worth it, and if you enjoy it you can leave a donation, but best of all you can enter for free (have to wait on a queue for a little while, but I think it is worth it taking in consideration all the things you can see inside). You can stay in there for as long or as short as you like, so it is a great time-filler and educational at the same time.