In Casper, be sure to spend time at the National Historic Trails Interpretive Center. http://www.blm.gov/wy/st/en/NHTIC.html
That's assuming of course that you do not exit the Interstate at Douglas.

If you do exit at Douglas you will be driving to Gillette through the Thunder Basin National Grassland, a quicker but more boring drive, unless you like grass. http://en.wikipedia.org/wiki/Thunder...onal_Grassland Oh, and in Douglas, be sure to visit the jackalopes. http://www.jackalope.org/