We chose to fly to Denver using Frontier airlines, I do not remember this airline even existing when I used to live in the US; So We are expecting a budget domestic operation.
We had lots of issues when trying to book/find seats from Washing D.C. to Denver, as most of the search engines were only using flights from IAD Dulles airport. These connections were not only at terrible times of the day, but also very expensive.
With a little luck and perserverence we eventually found some good connections from Reagan international airport.
Pre-book luggage or not
Frankly we could not understand any of the US domestic airline rules - so we decieded to pre-book check-in luggage as well as hand-carry.
Getting to the airport
The checkout from the Capital Hilton was super fast, and quick. Weliterally just walked out - as they had kindly pushed the hotel bill through the door at some point during the night.
From there we took a Taxi to Regan - it was my first time in an Hybrid car. Very impressive indeed, for sure the future. The Taxi driver told us he easily was getting 50 MPG - which has to be good for you pocket and the planet.