If you decide to come to Colorado start here: Plan Your Hunt | Colorado Parks and Wildlife
Since getting your father out west on an elk hunt is mentioned, what type of elk hunt you want will help determine the area. If your father is fit and wants to climb mountains the San Juans, Flat Tops, or any of the mountainous areas in central Colorado are open to you.
On the other hand, if high altitude hiking isn't desired, western Colorado is a better choice. Lots of oil/gas well roads that give access by vehicle to elk country. Great optics and Hunting Maps on a mapping GPS (available on this site) are essential equipment. Much of the area is steep pinyon canyons where a long range rifle can stretch its legs shooting across them.