Find great value on Frontier Airlines flights from STL to DEN when you book with CheapOair! Situated against the backdrop of Rocky Mountains, Denver is the second most popular county in Colorado. While in Denver, do check out the Colorado State Capitol. Constructed in 1890, it is the home of the Colorado State General Assembly and the offices of Governor of Colorado. Denver Firefighters Museum is another great attraction to visit. Built in 1909, this museum is known for showcasing the history of firefighting in Denver. It is home to four galleries that houses 30,000 photographs, manuscripts and artifacts. For delicious Mexican fare, visit Jack-N-Grill. It offers some of the delectable salsa, frito pies & Calabasitas, zucchini, bowls with squash and more.