I live in eastern Idaho so I can only say for this area. (Island Park, Ashton, Idaho Falls, Pocatello)
I have been hitting the Bear Gulch / Mesa Falls trails quite regularly these past couple of weeks as I just got my first set of skis for Christmas. The trails are pretty nice, but icy and bare in some spots. The weather channel has us pegged for a few feet of snow in the next couple of days though so that will change. Yesterday I took the eastern side of the trail and went out to the falls and back which is an 8 mile round trip. There are a few lookout areas on the west side of the trail. These trails are in-track only that intersect some with the snowmobile trails.
Here is the link to the trails around this area too. Caribou-Targhee National Forest - Winter Sports:XC Skiing/Snowshoeing
There are more trails in Island Park and Harriman State Park about 20 miles north of me that I have yet to hit this season. I see folks in the parking area in the morning gearing up to go out when I drive by for work though. AFAIK these are in-track trails.
And about 50 miles north of me there are the Rendevous Ski Trails in West Yellowstone, MT. I have been on these trails a couple of times and I have to say they are pretty sweet. Awesome grooming, and I like the terrain. These trails are both in-track and skate. Also seeing this town is the west entrance to Yellowstone National Park you could take a day and go into the park and ski those trails.