By Train
Use passenger station in Granby (GRA). No public transportation between train and park.
Bus
The Denver/Boulder area operates an excellent public bus system with many routes and frequent stops all over the area. You can easily travel from DIA to Lyons for about $25 per person (see rtd-denver.com for maps and schedules). From Lyons, Longmont, Boulder, and DIA, you can also ride with a private operator to Estes Park (estesparkshuttle.com, $85 round-trip from the airport), which is a bit more expensive but faster.
Taxi
For private taxi service in the Estes Park area, call Estes Park Taxi at (970) 372-9888 or Peak to Peak Taxi at (970) 586-6111.