Here are some resources for the upcoming Constitution Day on Monday, September 17th. The document was ratified on this date 220 years in 1787.
National Constitution Center -- interactive resources including a timeline, Founding Fathers info, clause-by-clause explanation of document.
This site also has a series of podcasts in the We The People Stories that go way beyond the boundaries of Constitution Day and would be useful for many other activities and research.
Although it is too late to order the free poster for this year, here is a lesson plan and a Bill of Rights game (elementary level).
Some of these and other helpful sights including the one from LC American Memory are also available here.
Here are some books that will also be informative and useful if you have access to them:
The Constitution / Finkelman c2006 Nat'l Geo Society
The Constitution / Pierce c2005 Abdo
The Bill of Rights / Pearl c2007 Picture Window Bks
La Constitucion / Armentrout c2006 Rourke
The Constitution: the story behind America's governing document / Graves c2004 Chelsea
Creation the U.S. Constitution / Nardo, ed. c2005 Greenhaven Press