Visiting friends in Boulder? There's an easy way to get there from Denver Airport. The RTD's AB SkyRide bus. I found the RTD's website a little confusing so I thought I'd document the trip here.
The AB bus is $13 each way which can be paid with exact change to the driver. There is also a round trip option for $24 that can be paid by credit or debit card from the RTD desk inside the terminal. For some reason, you cannot buy paper one-way tickets with your card - round trip only.
The RTD desk is inside the center of the main terminal.
Once you purchase your ticket head upstairs and look for door 605. (Currently due to construction they are only using the east side pick up location - thanks to Alek in the comments.) Once outside that door, cross the street to the island where the RTD boarding signs are. You can find the bus schedule on the RTD website.
Once the AB bus arrives, you can insert one of your purple tickets directly into the dollar bill slot on the bus money collector.
And, of course you'll have to find a seat before the inconsiderate TSA agents, just getting off their shift but still in uniform, spread out over two seats and pretend to sleep. All at 2pm in the afternoon. Their professionalism knows no bounds.