Do you want speed, or solitude? I-70 for speed, US50 from Cańon City to Montrose for solitude. Both are some of my most favorite roads. I-70 is gorgeous, but crazy busy.
Yeah, the stretch from Denver to vail is crazy depending on the year. Snow season will be busy with people going to ski. Summer season busy from people getting into the mountains. Like literal stop-n-go traffic for miles. So, #1 i would plan to hit that stretch on an "off" time.
Central city is a cool stop if you like gambling. Its like an old frontier mining town turned into a gigantic casino with lots of shops.
Glenwood Springs i would stop for a quick dip in the hot springs and get the best Taco joint in Colorado, Slope and Hatch.
Maybe take a quick detour SR65 through Grand Mesa (if its summer time).
Maob isnt far off I-70 too.