Best 3 National Parks to visit in CO
The phoenix mine is pretty cool and so see history of why people started to live in CO.
While you are in that area, you can go to St Amry's Glacier and hike up to it.
9am to 6pm isn't much time. This minimizes driving.
I would suggesst Leadville and Camp Hale, but those are about two hours from Denver (each way). Very scenic there.
Idaho Springs/Mt evans is close.