The National Park Service http://www.nps.gov/
has a very user friendly website packed with information about each and every National Park--including lesson plans for teachers, free materials, videos for download, maps, history & culture information, trip planning guides, science information, and an interactive Kids' Zone.
Have your students prepare presentations for a "National Park Day"....and invite the whole school to visit (i.e. similar to a science fair)....Make it a competition to see who can produce the most exciting, INFORMATIVE, National Park presentation (let the visiting students vote for their favorite presentation)! And most importantly, make sure all of the students' presentations have a geography component using The Five Themes of Geography as a focus for their research.
The Five Themes of Geography?
Location, Place, Movement, Human/Environmental interactions, Regions
This is one of many videos available on the http://www.nps.gov/ for download.