There are two iPad apps from the National Archives, one profiles a document per day, the other teaches you about what some of those documents mean. Both apps are free of charge.
This app allows you to see which document they have assigned per the date. You could have some fun with this, such as picking dates in U.S. history (July 4th or September 17th), or birthdays and other dates significant in the student's life to see what documents come up. For more, you have to click on the information link and go to their web site.
Link: Today's Document
This app is the interactive, teaching app that goes more in depth of how the documents impact our lives in the U.S. The challenge on the U.S. Consitution (it's a matching game between the documents and the articles of the constitution governing those documents) will be good for the older grades learning U.S. Civics.
Here is the link: DocsTeach