Very nice maps of North America by region or by states, in a variety of different levels of detail, are available from http://www.epa.gov/wed/pages/ecoregions.htm
If you ask nicely you can sometimes get a large classroom copy mailed to you.
One example of how to use this is to post them side-by-side with a geology map of the same area, or a precipitation map and have the students looks for similarities and patterns in the distribution of vegetation types and the surface geology. They can also look for patterns in the distribution of vegetation types and the amount of precipitation, or of any other weather characteristic you might be able to generate maps of. Shown is Oklahoma but these are available for most states and regions.