Some of the field trips that have been shown in the past are "Discovering the Mysteries of Bryce Canyon"; "Buffalo Soldiers: African Americans in the Frontier Army"; "Searching for Life in the Great Smokey Mountains"; and "Tails from the Tetons: Grand Teton National Park". There are several more listed that might be of interest to your class.
There is a new electronic field trip coming in February, 2011, "Gulf Islands National Seashore" that you can register for. You have to create an account, but registration is free. By registering for this electronic field trip, you and your students can call in or email to ask questions about what you are watching. What a great learning experience! Be sure to check out all of the electronic field trips that the National Park Foundation is offering. Great information, great activities, and great lesson ideas: best of all, it is all free!