If you are looking for a great day in Colorado for the whole family, you should try the Great Sand Dunes National Monument. The Dunes are about 785 feet tall and go on for miles. When you hike start early in the morning. It does get hot in the afternoon. Take lots of water. There is a creek at their edge. This is a good place to relax after the hike. The water flows in surges and feels good.
While I would normally suggest camping over a motel, I give this one area a waiver. You can follow the day at the Dunes with a night at the Movie Manor Motel. This is a unique place. All the rooms face the screen of a drive-in theater owned by the same family. They pipe the sound in to the rooms. Great concept! We found this place by accident when we got caught in a rain storm.
This is one of the hardest choices for my pages. Second only to my home State of Indiana. I could have gone with the light hearted Butterfly Museum the VW Bug out front, the Colorado National Monument, Mesa Verde, a saw caught in a petrified tree stump in Florissant, etc.