Turn the White House into a Taco Bell, the Eiffel Tower into the world's biggest slide, Big Ben into an astronomy tower. Those first two are just for chuckles. Oh, oh! The Palace of Versailles would be a giant mirror fun house! Why? Well, to eliminate overcrowding in prisons, of course! The prisoners would be placed in the center of this gigantic, and hazardous, maze, and those who make it out alive are given the treat of being thrown back into prison. Hey, they get to live, right? Natural selection at its best.
Ah yes, and I would build a mansion in Venice. Or Barcelona. Lovely architecture.