Find great value on Frontier Airlines flights from COS to DEN when you book with CheapOair! Take a trip to Denver for its cowboy mining past, vibrant art & culture and Front Range brewpubs. Also known as 'Mile High City', this city is located at an altitude of 5,280 feet/1,600 meters above sea level. Denver is the capital and the largest city of Colorado, USA. It is home to numerous attractions that include the Colorado State Capitol, the Denver Art Museum, the Black American West Museum & Heritage Center, Chamberlin Observatory, Denver Museum of Nature & Science, the Museo de las Americas and many more. Go for shopping at the 16 th Street Mall. It is home to stores like Veda-Renaissance Spa & Salon, Diamond Spa and Massage, Victoria Secret, Banana Republic, Forever 21, Claire's, H&M, GAP, Hollywood Cowgirl Bling and many more.