The U.S. Senate has posted material from the Legislative Branch: http://www.senate.gov/artandhistory/history/common/generic/ConstitutionDay.htm

The National Endowment for the Humanities provides content on the Constitution through the EDSITEMENT web site: http://edsitement.neh.gov/constitution-day

The Census Bureau's Statistics in Schools (SIS) program offers many activities to highlight Constitution Day -  at https://www.census.gov/programs-surveys/sis/resources/constitution-day.html.

C-SPAN hosted a discussion, "Constitutional Role of Judges: An Exchange Between Justice Breyer and Justice Scalia"