By the way, off road in the high country in December is not going to happen. So plan to stick to the pavement that time of year. We measure snow in feet, not inches. So rent a snowmobile, and leave the 4x4 in the parking lot. 4x4 trails that are open will be mostly nonexistent in the high country. There are numerous snowmobile rentals in the area, a few have snowbikes as well, if your into motorcycles.
If you want to try skiing, fly into DIA, take the train to union station, make the connection onto Amtrac to Winter Park. spectacular trip all by itself. The bus is free around winter park. You can get some learning days with instruction and equipment etc. I think you can get to Granby Ranch easily from there as well. Would have to check that out, but I think doable. Its a family friendly "easy" ski area with lots of vacation rental options.
If you want the Leadville area, as stated, highest incorporated city in north America. Highest Road in the North America (Mesquito Pass, closed in winter) and highest airport in North America. Home of Molly Brown etc. Not too far from Camp Hale. Home of the training grounds for soldiers in WW2 who learned to fight on snow skis. ski Cooper is there. But again, its winter, and the snow will be unpassable in a 4x4. Headwaters of the Arkansas river.
Remember the higher you go, its like going farther north. Winter is more severe, and longer, snows tend to be deeper and longer lasting, etc. The Arkansas river valley tends to be a banana belt. Beuna vista and Salida are nice communities that tend to have less severe winters, but close to 14,000 ft Collegiate peaks, so snow options are available and hot springs as well. There is a vibrant music scene in Buena Vista and Salida area as well as micro brew pubs, distillery's etc. Not sure how active that might be during the "off" season. Depending on the snow conditions,(and it varies greatly from year to year) and on how prepared you are, there might be some exploring done with a 4x4 in that area. Mostly unmaintained county roads, but still nice day trips. Just go prepared. I've spend enough all nighters searching for the unprepared.
This can be very unforgiving country, especially in winter.