Denver's light rail system was just recently expanded down the Southeast stretch of I-25 and the Union Station project will do more than reach the airport -- it's also planned to go the opposite direction to the Northeast (the only part of town where it's still hard to get downtown from) and include long-distance commuter rail lines from neighboring towns.

And Denver also has one of the country's best networks of bike trails. And RTD (Denver's bus and light rail operator) allows riders to take their bikes on light rail trains (you have to get on at the back of the rail car and stand), even at rush hour.