What a surprise!  Rocky Mountain National Park may be in the top three parks I have visited (as of the time of this writing).  The sheer size of the landscape is amazing and it is all very accessible on day hikes.  One of greatest hikes was a three hour hike above the tree line at over 9,000 feet - it wiped us out! Having also now visited the Canadian Rockies, it is amazing that in a single mountain chain that goes across multiple countries, they have a very different character about them. But both are incredible places to visit.