Some of the plowed roads in Yellowstone are open for just a short time every year to bike riders. This great opportunity to enjoy the road from West Yellowstone to Mammoth car-free lasts until April 15th, when the other park roads gradually open to motorized vehicles. You can enjoy the quiet, the animals, and all the beauties of the northwest part of the park on two skinny wheels. Click here for all the details. Note that the road from Madison to Old Faithful is closed at this time for bear management.
The same privilege is offered by Glacier National Park in northern Montana, another paradise. But Going-to-the-Sun Highway requires really ambitious bike riders with good brakes!