Building what I want would be heaven on earth for me. If I hadn't been such a total failure in maths I'd have become an architect. One of my favorite thought games is thinking up town-building projects. I'm also fond of designing arches and towers.
My more altruistic favorite idea is building a kind of education village and other surroundings that fit the wants of people according to their ages.
My more selfish favorite idea is to build a James K. Polk memorial, and it would be a sort of maze where you cannot get lost - with four or five towers there and arches galore.