Very soon, I'll spend some time in London. The weather forecast isn't that great and I think I have to expect some rainy days. Sightseeing in wet clothes is okay for some days, but not everyday. So I'm looking for some day-trips to interesting places which are suitable for bad weather. Any recommendations?

I'm interested in a lot of different things, but I'd particularly like to see things that I can't see anywhere else. Additionally, I like natural sights, landmarks and architectural sights as well as places that have a historical meaning.