One resource is the Library of Congress:
From home page, check under teachers/classroom materials/lesson plans/American history.
Other items on the site would also be helpful.
It’s always helpful to have another American history text from another publisher–especially on a higher level– available as a teacher resource to give extra information and additional perspective to topics.
If you have the time, browsing in a non-textbook American history book is valuable for fleshing out the topics covered briefly in any school text. A book such as Paul Johnson’s A History of the American People or Howard Zinn’s A People’s History of the United States would serve that purpose. Access to a collection of primary documents to occasionally read from is helpful, such as Zinn & Arnove’s Voices of a People’s History of the United States. For added dimension, browse in Gaustad & Schmidt’s The Religious History of America.
I personally enjoy the stimulating insights of essays by Daniel Boorstin, who explores American cultural history. One example is “Making Experience Repeatable,” in which he discusses the unintended consequences brought about by widespread use of the camera, phonograph, and recorder. The Daniel Boorstin Reader provides a broad array of his themes.