Find great value on Frontier Airlines flights from WAS to DEN when you book with CheapOair! Ranked 16 th of the most populous U.S. metropolitan area, Denver is the largest city of Colorado, USA. Visit the Molly Brown House Museum. This museum was home of the American philanthropist, activist and socialite, Margaret Brown. Keep your tiny-tots engaged by taking them out to the Denver Zoo. Spread across the area of 80 acres, it is owned by the city and the county of Denver. Opened in 1896, this zoo is home to species from all over the world. From primates to hoofed mammals, carnivorous mammals to pachyderms, birds to reptiles & fishes, you will find it all here.