From Estes Park, Colorado take Highway 34 west into Rocky Mountain National Park. Drive toward Trail Ridge road and keep your eyes open for the local elk herds among the apsens.
My wife and I were staying in Estes Park during the fall and had driven into Rocky Mountain National Park to view the elk at dusk. This pair of bulls was sparring in the distance, so I set my camera with a 70-200 lens and 2X converter up to photograph the action. The dominant bull later walked to within 25 feet and I was able to get some very impressive portraits of him before he once again charged off to defend his harem.