Here are some notes that I wrote about how to visit Boulder, Colorado.
Denver International Airport (IATA code DEN, but often referred to as "DIA") is actually twenty miles or so east of Denver. I usually fly there via Southwest, since DEN is a hub for them and they have low prices and fair cancellation and boarding policies, but all other carriers fly there too. If you have some time to kill in the airport, feel free to wander around and check out the weird iconography. I've heard that Root Down serves legitimately good food by the C gates, although I've never had time to stop there. I've eaten a passable brisket salad from Red Rocks Bar & BBQ in the terminal, though.
Once you get tired of hanging out in the airport, the RTD AB westbound bus runs from DEN's Transit Center (first stop) to the Downtown Boulder Station (last stop) about once per hour. (You'll also see the name "SkyRide" used to refer to all routes running to or from the airport.)
The airport layout confused me at first (and still does sometimes). The plane will drop you off at the A, B, or C gates (Southwest uses the C gates, United uses B). Follow the signs to Baggage Claim and Main Terminal; they'll lead you down two escalators to the free underground tram. It runs from C to B to A to the terminal. When you get off, you'll go back up two escalators and arrive in the huge, open terminal. Look for signs for the Transit Center and Westin Hotel; you'll exit through sliding glass doors at the south end underneath an enormous American flag.
Outside, go down the long escalator and follow the signs for buses; the AB bus picks up at gate 8. The bus stops are in a big, covered-but-open-air tunnel, so you might have a chilly wait if you're visiting in the winter. There are restrooms by the bus stop.
You'll probably see a group of people lining up for the AB ten or more minutes before it departs, especially when school is in session. The fare is $9 for adults and $4.50 for seniors. The driver will load baggage under the bus in reverse order of the stops (i.e. Downtown Boulder Station bags go in first). You need exact fare (paying for multiple riders at once is allowed), but the driver will usually make change using the money from other passengers. There's also an RTD counter in the terminal (you'll probably walk past it on your way to the exit) that I think may take credit cards, although it's closed often enough that I wouldn't count on it. If there's no traffic, the bus takes a bit over an hour.