When we lived in DF, we made a point of driving out to Valle de Bravo to witness the splendor of…read morethe Butterfly Sanctuary.
Each winter from months November to February, thousands and thousands of monarch butterflies fly hundreds of miles to this area of Mexico. You can hike or ride a horse (I think we paid $100 pesos per person) the 3-4 mile hike to the top of the mountain where as you gradually ascend, you start to see more and more fluttering orange wings until you reach the top where the tress are literally covered in monarch butterflies, nearby bushes moving with flecks of orange and black. Stand still and done will land on your head or your arm.
If you have the chance, go and see this miraculous sight of nature, it really was magical!