Today we went to the British Library. The tour there is very small, only one floor, but packed with some really cool things.
You can go and view some of the oldest Bibles in existence, original manuscripts from Mozart, the original copy of Lewis Carroll's Alice's Adventures in Wonderland, and tons of other things.
My personal favorite part of the Library was the Beatles shrine. They have the original manuscripts of "I Wanna Hold Your Hand" and "Yesterday" (as well as a few others) and you can listen to the songs while you look at the manuscripts. Something about the whole experience put a goofy smile on my face.