Here's a route that starts in Denver and ends in Denver. A bit more traffic, especially from Boulder to Estes Park but I doubt you'll find a more scenic ride. The route has a glitch in it, however. Google maps won't let you map a route over Loveland Pass and down Clear Creek. Go through Keystone over Loveland Pass to Georgetown then follow I-70 along the frontage roads and bike paths to Floyd Hill. Return to Denver along US40.
If you do the route for a car you get
this one. 407 miles