In Estes Park, take the road thru rocky mountain national park, hit grand lake, continue down thru Berthold pass to I-70. If you go west a little you will go thru the eisenhower tunnel and the continental divide. From Silverthorne/dillon/Frisco take 9 south past breckenridge and over hoover pass. to 285 in fairplay. That will take you thru buena vista and salida, and over poncha pass. the turn onto 17 is easy to miss, but that will get you close enough to see the signs. If you want to experience an old fashioned steam train ride, a little further south in antonito is the cumbres and toltec scenic railroad. its a beautiful trip up mountain passes and tunnels. I've lived in Colorado for 2 1/2 years now and still haven't seen all the great sights. I've traveled all of these roads and recomment them.
Have a great time!