A few personal opinions.
Trail Ridge is spectacular scenery and not to be missed. But it's a putting along road, not a "riding" road. Avoid summer weekends at all costs. Weekday mornings are best, be prepared for cold temps up high, maybe even snow or ice.
As someone who lives here, I like the less well known stuff. Lizard Head Pass is a great alternative to the Million Dollar Highway. Great scenery, but since it's down in the valley, great riding as well. Terrific sweepers.
My absolute favorite "hidden treasure" is 145 from near Gunnison down to South Fork, or vice versa. Amazing variety of scenery. Along the way Lake City and Creede are cute and relatively low key.
North of I-70 I like 131 from Wolcott to Steamboat Springs, maybe over to Walden and 125/40/9 back down to Dillon.
Note that all these are very scenic, but not the jaw dropping stuff on Trail Ridge or the Million Dollar Highway. The tradeoff is "ohmigod" versus great riding, and much less traffic.
Want an offroad adventure that most anyone can do? Cottonwood Pass from Buena Vista towards Crested Butte has a nice unpaved, wide, and smooth, bit on the west side of the pass. If that was fun, Keebler Pass from Crested Butte west is only a little harder. NOT SCHOFIELD, which is seriously difficult/dangerous, even on a dirt bike. In general don't go offroad in Colorado without specific knowledge/local inquiry/good weather.
Feel free to PM me if you want more ideas, after you get a basic idea. I've done 95+% of the major paved roads east of I-25, most more than once.
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