After you have taught a lesson on the separation of powers, reinforce it with a sorting activity. Make a sheet with a number of powers on them, such as "Can veto legislation," "Can declare laws unconstitutional" or "Conducts hearings." Photocopy the sheets. Distribute them to students along with a separate sheet with three columns, one for each branch of the government. Students must use scissors to cut out the powers and glue them into the appropriate column on the second sheet.
Divide the class into three groups, each representing one branch of government. Read a situation out loud that would involve one branch of the government. Examples include a state suing the federal government and a judicial vacancy that must be filled. Each group must confer and decide if it wants to claim the situation as its to handle. Groups must give evidence as to why. Give a team two points for a correct claim, and take away one point for an incorrect claim. The team with the highest score at the end wins.
This activity is a little more complex and might take a few tries for you class to get it right. Divide the students into the three branches of government. Present a piece of legislation to the legislative branch, such as a law banning cell phones from public schools. Have the legislative branch debate and vote on the law. If it is passed, give it to the executive branch to either veto it, or come up with a plan to enforce it. Have the judicial branch members debate with one another about whether it is constitutional. If the legislative branch defeats the bill, have the executive branch make arguments to the judiciary about why it is constitutional and necessary.
This activity also involves dividing the class into the three branches of government. Give each group a copy of the Constitution. Present each branch with an opportunity for an unconstitutional grab for power, such as the president seeking a third term or the courts denying the vote to senior citizens. The other teams have two minutes to find evidence in the Constitution for why the power grab is unconstitutional. Each time a team finds the proper evidence, award 10 points. Award 5 points to a team whose power grab goes unchecked.