Hi Lisa,
Check out Escalante National Monument, Bears Ears, or Canyonlands National Park in SW Utah, or Colorado National Monument/Black Ridge Canyons Wilderness in CO, but keep an eye on the temperatures. I’ve had below zero F nights there in Jan/Feb, but Nov/Dec have generally been in the 20s or higher. Colder down in the canyons where the Colorado River flows.
For mountains: the Pecos Wilderness E of Santa Fe and Great Sand Dunes National Park in CO can get some lucky seasons (little early winter snowfall) where trails even up to 9k+ can be done w/o snowshoes in December.
Rocky Mountain National Park trails are also usually quite compacted (hikers, snowshoes, and skiers keep them pretty spikeable) within a long days’ walk of the trailhead, but you’ll have to contend with some wind there. Bitter cold doesn’t *usually* arrive in the Rockies until January, but fall/early winter wind chill temps there are the real deal. The scenery might be worth it!
And don’t overlook the Oregon Desert Trail or Oregon Coast Trail or Lost Trail (CA) or the wilderness beaches in WA. I’ve been on all of these during the winter and while the storms can be totally outrageous (watch the weather forecast), it’s something special to be in those places during the winter months.
Mojave and Death Valley are phenomenal places to hike in the winter. Both can experience serious winter winds, but they’re usually easy to forecast with weather apps like Windy.com or Meteoblue.
If you’re good with snowshoes, your options open up: the central-to-southern PCT in the Sierras (CA) are pretty warm in December, as are the southern Cascades in WA and OR (although higher chance of rain/wet snow).
The Wyoming, Colorado, and northern NM Rockies get some killer winds that time of year. NoCA, NoWA, the central OR Cascades, and MT/ID get quite a lot of snow. WY, MT and ID Rockies can be incredibly cold if an Arctic front is coming down…
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