Several activities here will help your students further understand how the branches balance each other out. If you only have time for one activity, I suggest number 4, which uses primary source documents that your students will use to decide which branch is over-reaching and which is stepping in to stop it.
A wonderful worksheet that includes more details than most that I have seen. It would be great to use to review the lessons. You could also use it to have students identify the actual people who hold each of the offices.
A stand-alone activity that covers the topic completely and will assist your students learn more about our government. There is a video explaining the three branches, some quiz questions, and some suggestions for further research and discussion. You could also just use the video to add to your lessons.
A wealth of information can be found on this site with a great variety of resources. You could easily create a webquest or quick research activity for your students using only this site. It also includes a quiz and review activities.
Use this lesson plan that teaches the students the history of the federal government and the division of powers between that and the state governments and includes links to other resources on related topics for an excellent overview of the subject.