Grand Canyon and the Lincoln NF may very well may have snow by December; it varies from year to year. The amount and likelihood of snow will depend upon the altitude; above 7000 feet, snow is common by December.Again, it varies from year to year, so pay attention to the weather report.
I would recommend the Gila Wilderness (Gila NF), the Chiricahua Mountains in S Arizona (at least the lower reaches).
The West is a wonderland, full of interesting and intriguing locations, whatever your tastes may be. You can spend your life here and never see it all (spaking from experience!!)
one or more gifts or other benefits from the co-op.