Once a site where the U.S. Army manufactured chemical weapons such as mustard gas and GB nerve agent, and later leased to a private enterprise to produce pesticides, the Rocky Mountain Arsenal has become an environmental success story. The 15,000-acre site, an area of open grasslands and wetlands just west of Denver International Airport, is home to more than 330 species, including deer, coyotes, prairie dogs, birds of prey, and bison. This is one of the country’s largest bald eagle-roosting locales during the winter. The Rocky Mountain Arsenal Wildlife Society Bookstore is at the visitor center, and there is a scenic 9-mile drive and about 10 miles of hiking trails as well as catch-and-release fishing. Allow at least an hour.