Monarch Ski Area is an operating ski area all winter long. That said, it's on national forest land, so there's no reason you can't fat bike here... especially when the lifts aren't running. As an added bonus, there's a relatively easy way to reach the top of the mountain by pedaling up the highway, turning right onto the snow on Old Monarch Pass Road, merging onto the green runs when the road hits the edge of the rest, and then follow the easiest slope to the top of the mountain. Once there, you can traverse to the other side, or pick one of the runs before you get there. The choice is yours!!
CO
http://www.singletracks.com/item.php?c=1&i=10078