Tower of London
If you enter the Tower of London exhibition, a lift whisks you up four storeys to the walkways at the top. From there you can learn about the Tower's history, but I was just there for the fantastic views of the city. I think my photo of Butler's Wharf would make a fantastic jigsaw!

After the walkways, you exit the tower and use the second portion of your ticket for the Engine Rooms down the road, where you will find such wonders as "Mud discharge" pipes.

Click on any of the images below to see a larger version.