Yosemite, in February when the fire waterfall appears. The spectacular view of the waterfall is created by the reflection of sunlight hitting the falling water at a specific angle. This rare sight can only be seen at a period towards the end of Feburary.
Best article so far on hikes from Estes. K 12 incredible hikes, lakes, and viewpoints you can't miss on your first visit to Rocky Mountain National Park. Travel the highest highway in the United States to over feet above sea level!
Yellowstone National Park Guide: America's 1st National Park
For me, going to the Yellow Stone National Park would be for viewing the Twin Falls and see the diverse amount of wildlife that was protected. It will be a pleasing experience and one I will take to my heart.