I teach 5th grade and use this website all the time. I'm sure the K-2 version is just as good.
http://bensguide.gpo.gov/k-2/index.html
Also there is Congress for Kids. May be a little harder, but if you use it together it may work.
http://congressforkids.net/Constitution_index.htm
The Center on Congress is a great site for older children, may give you some background information.
http://www.centeroncongress.org/
As for activities I have had my students make mobiles to visualize the balance between the 3 branches. I collect wire hangers from the cleaners and have some photos that I took of the buildings. They must tell who works in the branch, where they work, what the responsibilities and other information (I cannot think right now and the project is at school.)