Climb the Dome of St Paul's

Cathedral Floor
+ 257 steps, Whispering Gallery
+ 376 steps, Stone Gallery
+ 528 steps, Golden Gallery

We weren't allowed to take photos inside St Paul's but here is a pretty good virtual tour. I recommend checking out the view from the Whispering Gallery - its only 30 metres up and the chairs are already ant-like. Oh and there's the ceiling...