Once students understand the basic ways that the U.S. Government works, we can make the classroom into a simulation for students to practice. Every two weeks students can move to a new branch of the government and learn about it.
This is a great project for G+T students in fifth and sixth grade. Variations I have done include working individually and letting the students choose anywhere in the world they would like their country to be. The geography and climate of their portion of the earth affects the other aspects, such as history, customs, foods, etc.